'Siblings' is more appropriate to use.
The Italian word for brother is "fratello" and for sister is "sorella".
There is no single word in the English language for your sister-in-law's brother.
brother
The French words for brother and sister are frere (brother) and seour (sister). There is a popular children's song which has "frere" in the title. It is called "Frere Jacques". This song is known in English as "Brother John".
Siblings you dumb people
Siblings!
brother or sister
"Soror" means sister. "Frater" means brother.
siblings
Insest
The word for niece in Portuguese is "sobrinha" if she is your brother or sister's daughter, or "sobrinho" if he is your brother or sister's son.
Your niece is the daughter of your brother or sister.