I know this is super hard, but it is really important to have the conversation with her!!! Especially if you ever will consider leaving the girls at her house alone!
An idea on how to start is to say something like this. "Grandma, I am very upset right now. I have found out that you are communicating things about our life to my ex, showing his picture to the girls and ___________" This is not ok. You are someone that I trust and I need to be sure that sending the girls here is SAFE. When you keep reminding them of him it is hard for them to heal emotionally. When you mention things to him or his mom, then it just stirs up drama - and we don't need that, we are trying to heal. I love you and you having a relationship with you is important to me. Its also important to me that the girls have a relationship with you. But I will not allow this to continue. I need to have assurance from you that this WILL not continue, or I will have to set up some boundaries about when/how the girls and I spend time with you."
It might work better to write a letter, I know that can seem like a cop-out, but it will assure you that you communicate ALL the points you want to make, and that she hears them all, AND that you don't chicken out. :) If you feel its best to do this face to face instead, then write it all out and maybe condense it down to a short list of key words/ideas and take that list with you so you make sure you cover everything.
It is important to listen to your husband on this one. Someone outside the family sometimes can see the inter-family relationship issues better. Your relationship with him and your daughters is primary to everyone else, so whatever it takes to be on the same page with your husband (within reason of course) should be done. The other family member relationships are important, but only if they are emotionally supportive of your core family unit.
It does not sound like your Grandma is supportive of your family, she obviously wishes you were with your ex instead. I would ask her point blank if that is what she desires. She needs to be honest with her feelings as well, because that is the only way to heal this relationship. There has been betrayal with your family, and it sounds like you are trying to be the bigger person and reconnect with them. That is admirable, but in reestablishing relationships where there is betrayal, there need to be clear boundaries to help ensure that you don't get walked on again. It might be emotionally easier to just be nice to everyone and accept them again. BUT, there will be the underlying issues and they will keep creeping up. Listen to your husband, he's seeing stuff that you might not have noticed (maybe you're just used to living like this and its a "normal"), but its really important to deal with it!!!
Best wishes - this sounds really hard, but I know you will be thankful you did it in the end!!!