grandparents and stepkids

My grandparents and I used to be really close, but I haven't seen them since I got married bc they live so far away (they live in  IN and I'm in NYC). They are in their 90's and won't be able to travel much longer for obvious reasons, however they are coming to ny to visit my parents on Labor Day weekend. My husband, our son and I are all going upstate to see them as well, but my 12yo stepdaughter's mom won't let her come too. I'm seriously afraid this will be the last time for her to meet them... I'm not sure what to do. In my opinion she's part of the family and should be there and meet them, but even my husband says it's not that big of a deal. I'm broken to think they might not ever meet her if we can't make it out to IN as a whole family. Idk what to say or do, or who is right or who is wrong.