I need help with my step kids mother!

Hi ladies, I'm really hoping some of you can share some thoughts on this for me, as I really don't know what to do... I'm sorry this is going to be a long one!

So I've been with my fiance 2 years. Early on in the relationship (I'm talking 2 weeks in!) His ex, mother of his kids, (10, 5 and 1) demanded that he tell the kids about me, otherwise she would tell them that Daddy replaced them and has a new priority now!

Backup... So the reason they split in the first place, was because he found out that she was cheating on him, so he left, staying at the house at times as she would go out and not come home for hours/days. Even to the point he would have to miss work because she wasn't home still.

Anyways, so she said that it would be easiest to meet the youngest first as it would "damage" her less 🙄 which I did, we went for breakfast, my fiance picked the little girl up (other kids at school) and the ex was being weird, asking for kisses and cuddles and asked him why he didn't love her anymore... Erm coz you cheated on him! All through breakfast she was calling him, screaming down the phone, even saying she would kill herself! All of a sudden she burst into the restaurant, threw the plates across the room and tried to hit me!! She screamed at me that I would never replace her, snatched the little one and tried to take the buggy down the stairs!! My fiance went after her. After a while, he came back and said she wanted to apologise. So she says "sorry I threw plates at you, but if you ever think you can be my daughter's mother! He will get you pregnant and leave you. He will get you into debt. He will cheat on you!" I just held out my hands and kept saying ok. My fiance said that's enough and we left. After this, he gets a message from her mother saying that she's gone to sort her head out and he needs to sort the children, if he can't, he's needs to call social services! So obviously he took the week off work to look after them, not knowing when she would be back. (He lost his job because of this in the end) when she did come back on the scene, she was awful! Incident 1.

Fiance borrowed my car to drive to work. Long story short, she drives into the back of the car, with the youngest in her car! He pulls over and she's screaming at him, in hysterics, so he tried to take the child and she's hitting and punching and kicking him and the car. Incident number 2!

So what follows this is nearly two years of massive periods where she refuses to let him see the kids, months at a time. Police have been involved, mediation company's have been involved but she never returns the paperwork or the company's keep delaying things. Obviously the situation is so much more complicated, but I'm trying to give you all a rough outline of the things that have happened!

So currently. She is communicating with my fiance's sister only. Out of the blue, last week, she asked when my fiance wanted to see the kids. They agreed we would have them last night, and they would have the option to stay tonight aswell if they wanted/felt comfortable (we have just moved into a place together)

So all today, she has been messaging his sis, asking for me to go with my fiance to return the kids. She's asked if I can unblock her on fb. The kids called her and she she was so sickly sweet about me.

My question is, are we wrong to be so suspicious!? Something about this just doesn't feel right. Obviously she's had mental health issues in the past, and it's totally possible she's had a few drinks today (obviously well within her rights if this is the case) but how do we respond? On all honesty, I'm frightened to see her face to face. I'm the least confrontational person there is! At the beginning, I was so rational towards her, reminding my fiance to see things from her side of the fence, understood how hard it was to let your kids be shared with a stranger, I totally got all that. But she has been so MEAN to me and my family, when all I've ever done is loved those kids! And seriously, I love the kids! I've always wanted to be a mum and I've been struggling with infertility for years.

What should I do??