[ColdInCanada11]

I think you will be fine without being "adopted"- yes, some people are "OMG BESTIES FOR LIIIIIFE" with their big sister, but more often than not, it's just a regular friendship. The only times I have seen the first type happen on a frequent basis is when the little sister is a first year and the big sister helps them with not only the sorority but university in general. You are already good on that front, so (as several people have said), focus on your little sister. Some of my best friends from my chapter have nothing to do with my family tree- if I had been deadset on spending allllll of my time with my sister-mom, I wouldn't have that relationship now.