Excluded From Best Friend`s Wedding Party...

My best friend was my maid-of-honor 5 years ago, and now she is engaged. Internally I assumed that I would probably be in her party, but refrained from saying anything out loud about the wedding details, since I know it can be an overwhelming time trying to plan everything. A few days ago she told me I cannot be in her wedding, because she is picking her two sisters, and her fiance is picking his two brothers, and he cannot think of anyone else to include on his side, so they want to keep it 2 and 2.
Granted, ultimately this is her wedding, so she is free to do what she pleases, and I also don`t need to be in, or not to be in a wedding party to know that she and I are best friends - our relationship won`t change. But for whatever reason, I still feel a bit slighted.
I suspect there might be more to this - our ethnicities are different, and perhaps her family/in-laws don`t want someone of a different race in the party. I also know that she and one of her sister`s do not get along - I have heard plenty of stories and tears over that. However, blood is thicker than water.
In either case, I am kind of hoping that that could be it, and not for the sheer reason of making things symmetrical. Would it be improper for me to inquire about this in the future? She and I share EVERYTHING sensitive, so if it is a familial or ethnic thing, there`s no reason why she wouldn`t hesitate to tell me - she knows I wouldn`t blame her for it - obvious she`s getting pressure from her family. If the only reason is because her fiance can`t pick a third person, I personally think it`s a lame reason, and it`s hard for me to change my mind about that. Any ideas on how to handle/rationalize this internally/externally are welcomed.