Siblings

So my SO's little sister could pass as our daughter. I love that little girl so much and apparently loves me and misses me whenever I'm not around. Does anyone else have a relationship like that with their SO's siblings. I'm an only child but I consider her my little sister now. We joke that when she goes through her first break up I'll be the one to take her shopping while the brothers take out the guy. I love that his family has become my family.