Who is your SIL?

[Aparecium]

Or BIL?

I know your DH/DW's siblings are your SILs and BILs, but what about your sibling's DW/DH? Are they also your SILs/BILs?

Would your DH's aunt be your aunt-in-law, or is that not a thing?

i also class my brothers wives as my SILs as well as husbands sisters

My brother’s wife is my SIL. I also have a BIL who is DH’s brother. I call BIL’s wife my SIL even though she is DH’s SIL and really my SILIL!

I call DH’s cousin’s kids my nieces and nephews even though they aren’t.