NEED ADVICE!!

So to give a little back story, I use to be in a very toxic relationship with a guy whom I had my first child with (now 2) and ended up getting pregnant again by before we split (due October), the relationship lasted about 3 1/2 years. Since the spit my children’s biological father has made little to no effort to be in their lives other than a phone call once a month or every other month and him and I have cut complete contact with each other due to how he speaks to me and treats me every time we do talk. The family on his side has absolutely nothing to do with the children except for his mom who doesn’t have a whole whole lot to do with them but tries to make an effort to still be a part of their lives. I am now engaged to an amazing man, he treats me amazing and he treats my children as if they are his own, he takes care of us emotionally, physically, financially, he has been a blessing to us to say the least. Recently my fiancé and I have been having some issues because I still have some what of a relationship with my children’s biological grandmother on their dad’s side. He is not comfortable with me having a relationship with her outside of it concerning the children. Some reasons why he feels this way: He believes she should be trying to do more to make her son be a better father and stay on his butt about it, he doesn’t believe she has enough to do with the kids and believes she should be doing more herself to be a part of their life and help out more, he is afraid she would turn against me if the kids bio-father was to take me to court, (the kids bio-father posted on Facebook about the whole situation and told a bunch of lies about me and the situation and instead of sticking up for me or anything she commented on it telling him to get a lawyer, I asked her about it and she said it was more of a jab towards him but I don’t know) so my fiancé doesn’t think she’s as much of a friend to me as she says she is, she has crossed some lines when it comes to being respectful to the boundaries I have set concerning the children and their bio-father, etc. Her and I have always had a pretty decent relationship, it’s has become a bit rockier since her son and I split but I still feel like I have some kind of relationship with her (other than just for the kids sake) and my fiancé just doesn’t understand that and isn’t comfortable with it and it is causing some major issues in our relationship. I have explained to him how I feel but he doesn’t think I should have a relationship with her outside of the kids because of all the reasons listed before and he says I shouldn’t act like she’s my mother in law anymore because she’s not. What should I do? What would you all do in this situation? Thanks to anyone who took the time out to read and comment 💖