I pledged in January of my freshman year and received my first choice as a Big Sister. She was only a sophomore, but that spring she announced that she was getting married that summer and not returning to school. I don't think her husband was particularly enthusiastic about the whole sorority thing because she didn't even come to my initiation.

HER big sister stepped in and made my initiation week great. I literally don't think I ever saw her again after her wedding. During the next three years, I sometimes was envious of those girls who were close to their big sisters, and thought about how I could possibly have had someone different, if I had put another person first on my list. But that's just how it was, and some girls who had seniors as a big sister didn't have Bigs in the chapter, either. I got a little sister my sophomore year and was able to be there for her and have the relationship that I had wished that I would have with my Big Sister.
ngsu, my suggestion is to focus on the relationships that you DO have in the chapter. No need to cause awkwardness and drama.
BTW, I want to add that my big sis was a really great person and I don't hold it against her that she wasn't there for my initiation week. I really don't think her husband would let her come. I got the impression he was pretty controlling, and even as a 19-year old, I saw that. Pretty sure they eventually divorced. I wonder where she is today.