English has neutral words for mixed family groups: children, siblings, grandparents, parents-in-law. Italian mostly does not. It uses the masculine plural of the male word instead. Figli can mean sons, but it is also the normal word for children. Fratelli can mean brothers or siblings. Nonni is the usual word for grandparents, and zii can mean uncles or aunts and uncles together.
This trips up English speakers in two ways. When you hear ho tre fratelli, you may picture three brothers when one of them is actually a sister. When you speak, you may look for a separate word for siblings or for children of both sexes, and there usually isn't one. You do not need one. The masculine plural does the job.
Word list
adultomA1adultgli adulti = adults of both sexes, or adult men; le adulte = adult womenbambinomA1childi bambini = children or boys; le bambine = girlsfigliomA1childi figli = children or sons; le figlie = daughtersfigliomA1soni figli = children or sons; le figlie = daughtersfratellomA1brotheri fratelli = siblings or brothers; le sorelle = sistersgenitorimA1parentsi genitori = parents (mother and father together)nipoteA1grandchildi nipoti = grandchildren or grandsons; le nipoti = granddaughtersnonnimA1grandparentsi nonni = grandparents or grandfathers; le nonne = grandmothersnonnomA1grandfatheri nonni = grandparents or grandfathers; le nonne = grandmothersragazzomA1boyi ragazzi = young people or boys; le ragazze = girlsziomA1unclegli zii = aunts and uncles, or uncles; le zie = auntsbimbomA2babyi bimbi = little kids (boys and girls), or little boys; le bimbe = little girlscuginomA2cousini cugini = cousins of both sexes, or male cousins; le cugine = female cousinsnipoteA2nephewi nipoti = nephews and nieces, or nephews; le nipoti = niecesnipoteA2niecei nipoti = nephews and nieces, or nephews; le nipoti = niecesparenteA2relativei parenti = relatives of both sexes, or male relatives; le parenti = female relativesprincipemA2princei principi = prince and princess, or princes; le principesse = princessesfidanzatomB1fiancéi fidanzati = engaged couple, or fiancés; le fidanzate = fiancéesgemellomB1twini gemelli = twins of both sexes, or twin boys; le gemelle = twin girlsneonatomB1babyi neonati = newborns of both sexes, or baby boys; le neonate = newborn girlsantenatomB2ancestorgli antenati = ancestors of both sexes, or male ancestors; le antenate = female ancestorscognatomB2brother-in-lawi cognati = in-law siblings, or brothers-in-law; le cognate = sisters-in-lawdiscendenteB2descendanti discendenti = descendants of both sexes; le discendenti = female descendantsfigliastromB2stepsoni figliastri = stepchildren or stepsons; le figliastre = stepdaughtersfiglio di mezzomB2middle childi figli di mezzo = middle children (boys or girls); la figlia di mezzo = middle daughterfiglio unicomB2only childi figli unici = only children (boys and girls); le figlie uniche = only daughtersfratellastromB2stepbrotheri fratellastri = stepsiblings or stepbrothers; le sorellastre = stepsistersfratellastromB2half brotheri fratellastri = half-siblings or half brothers; le sorellastre = half-sistersmezzo fratellomB2half brotheri mezzi fratelli = half-siblings or half brothers; le mezze sorelle = half sisterspadrinomB2godfatheri padrini = godparents or godfathers; le madrine = godmotherssuoceromB2father-in-lawi suoceri = parents-in-law or fathers-in-law; le suocere = mothers-in-lawbisnonnomC1great-grandfatheri bisnonni = great-grandparents or great-grandfathers; le bisnonne = great-grandmothersfigliocciomC1godsoni figliocci = godchildren or godsons; le figliocce = goddaughtersorfanomC1orphangli orfani = orphans of both sexes, or male orphans; le orfane = female orphansprimogenitomC1first-borni primogeniti = first-born children of both sexes, or first-born sonsvedovomC1widoweri vedovi = widowed people (widows and widowers), or widowers; le vedove = widowsconiugeC2spousei coniugi = husband and wife, the married couple; il coniuge = spouse of either sexsposomC2spousegli sposi = bride and groom, or grooms; le spose = bridestrisavolomC2great-great-grandfatheri trisavoli = great-great-grandparents; le trisavole = great-great-grandmotherstrisnonnomC2great-great-grandfatheri trisnonni = great-great-grandparents; le trisnonne = great-great-grandmothers
How it works
In Italian, a group that includes at least one male is grammatically masculine. Family words follow this rule, so the masculine plural covers mixed groups. Only the context tells you whether the group is all male or mixed.
Here is the rule of thumb. The masculine plural (i figli, gli zii) can refer to men only or to a mix. The feminine plural (le figlie, le zie) refers to women only. When a speaker means a mixed group, they simply use the masculine plural and add details if needed.
Ho due figli: una femmina e un maschio.
(I have two children: a girl and a boy.)
Hai fratelli? Sì, un fratello e una sorella.
(Do you have any siblings? Yes, a brother and a sister.)
The words
Children, siblings and the next generation
Figlio and fratello are the everyday cases. The step and half forms work the same way: figliastri can include stepdaughters, and fratellastri can include stepsisters or half-sisters. Gemelli can describe a boy and a girl born together. Nipote is a special case, because the singular is the same for both sexes (il nipote, la nipote). It means grandchild, nephew or niece. I nipoti can therefore be grandsons and granddaughters, or nephews and nieces. Pronipote works the same way.
Mia madre ha cinque nipoti: tre maschi e due femmine.
(My mother has five grandchildren: three boys and two girls.)
I gemelli si chiamano Luca e Sara.
(The twins are called Luca and Sara.)
Older generations
Nonno gives nonni, the normal word for grandparents. Further back you have bisnonno (great-grandfather), trisnonno and the more formal trisavolo (great-great-grandfather). Each plural covers both sexes, as do antenati (ancestors) and discendenti (descendants).
I miei nonni abitano a Napoli.
(My grandparents live in Naples.)
The wider family
Zio, cugino, cognato and suocero all follow the pattern. Parente (relative) is like nipote: one form for both sexes, and i parenti for a mixed group.
A Natale vengono tutti gli zii e i cugini.
(All the aunts, uncles and cousins are coming at Christmas.)
Domenica pranziamo con i miei suoceri.
(On Sunday we're having lunch with my parents-in-law.)
I miei cognati vivono a Milano.
(My brother-in-law and sister-in-law live in Milan.)
Couples and other groups
A plural often names a man and a woman as a pair. Gli sposi are the bride and groom, i fidanzati are an engaged couple, and i principi can be a prince and princess. Genitori (parents) is used the same way. Outside the family, bambino and ragazzo also follow the pattern: i bambini are children and i ragazzi are kids or young people of either sex.
Gli sposi escono dalla chiesa.
(The bride and groom are coming out of the church.)
I bambini giocano in giardino.
(The children are playing in the garden.)
Watch out
A masculine plural can include women, but it does not have to. I miei fratelli may be all brothers. If it matters, Italians make it clear with words such as maschi or femmine, or by naming the people.
Ho tre figli maschi.
(I have three sons.)
For an all-female group, switch to the feminine plural. With nipote and parente only the article changes.
Le mie nipoti sono tutte e due all'università.
(My two nieces are both at university.)
One more everyday shortcut: i miei, with no noun, usually means my parents.
Stasera ceno dai miei.
(Tonight I'm having dinner at my parents'.)
Summary
For a mixed family group, use the masculine plural: figli for children, fratelli for siblings, nonni for grandparents, zii for aunts and uncles, suoceri for parents-in-law. That plural can also mean men only, so read the context. Use the feminine plural only when everyone in the group is female.