One of my SIL's refers to me as her sister. It's very obvious if you look at us that I'm not biologically her sister :-) I think this is very sweet and my kids love it. I haven't asked her about it- I don't know why I feel shy about this one! Her parents immigrated to the US from Central America and her first language is Spanish. My kids speak Spanish. However, my kids' education hasn't covered "what you call your sister-in-law." I've wondered if this is cultural for her and if I should refer to her as my sister when she's with me? She is a very warm, affectionate, giving person. I sometimes wonder if I come off as colder to her as I'm not as outwardly affectionate as she is. I know this is a trivial question but it's been on my mind for a while.