'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
brother or sister
Siblings!
"Soror" means sister. "Frater" means brother.
The Chinese word for sister is "姐姐" (jiějie) for older sister and "妹妹" (mèimei) for younger sister. The word for brother is "哥哥" (gēge) for older brother and "弟弟" (dìdi) for younger brother. These terms reflect the age hierarchy commonly acknowledged in Chinese family relationships.
Insest
siblings
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.