BM help!! (confused-vent)

One of my BM was my CLOSEST friend for YEARS.....she actually introduced me to my FH one night when she met him at a bar we went to.....anyway..we were teh closest friends and she started hanging out with this other girl i dont really care for..but we all got along great....well apparently they started a relationship....(big shocker for me) and it's over..i believe it a phase....but she still spends literally every moment with this girl and started hanging out with me less and less...now that i have rob...my FH, forget ti we NEVER hang out..i think twice this past year...and she didnt even make it to our engagement party! well i had asked her last june to be a BM...she of course said yes....since then its like pulling teeth to get in touch with her....she doesnt give a crap about anything......but i really wantd her to be a part of our wedding...especially since she was the reason we are even together. the only time i speak to her is when she calls to complain about this other girl ( who is JUST a friend now..thats over) but i can NEVER get a hold of her otherwise. its KILLING me. my sister (MOH) says dont say anything anymore and just let it go....see if she asks or says anything by the time for dresses and all. But i would of course invite her but i would feel HORRIBLE not having her as a BM....but she has become so self absorbed.....and i could NEVER just flat out say something to her..she would get so upset..ahh sorry i dont knwo what to do...i mean i DO have a year and a half still....but she has become so hopeless......