Who is right in this argument?

Long story short, my mom doesn't trust anyone. My step-dad wants to spend Thanksgiving with his only daughter and granddaughter, but it would have to be with his ex-wife as well (they are really good friends, nothing more). My mom has six kids, and over fifteen grandchildren. She wants him to spend Thanksgiving with our family. He would love for us to spend Thanksgiving with his daughter and their family, but his ex-wife does not want to meet my mother (for whatever reason). Anyway, so he was going to spend Thanksgiving Day with his daughter and that group (he'd leave that morning, be back before dark), but my mom is having a hissy fit over it, saying that he's not right, and that her family would never do that to him. My mom goes to see her kids all of the time (my step-dad hardly ever sees his daughter), and I think she's being a total hypocrite. I'm caught in the middle.. help !