Advice needed Trigger warning - grooming/neglect/sexual abuse

I genuinely don't know where to start with this and know that it is so complicated that it may be too difficult to explain it all effectively but I'm gonna give it a try cus I really need to talk to someone tonight.

To start, I am a 37yr old woman from the UK. I live with my husband, who I've been with for 11 years and married for nearly 6 years. I have a 17 yr old son from a previous relationship and he has a 15yr old son and a 13yr old daughter from a previous relationship. We were talking online and then meeting each other for about 10 months before I met his kids. At the time he used to only have them at the weekend. I have always been very accepting of his kids, I class them as my own and have always treated them as such. My husband also adopted my son at his request, as his biological father has never wanted a relationship with him( his loss!). Not long after I first met his kids, he had to take full custody of them as their mother's behaviour had become very erratic and the kids were being put at risk because of it. At that point they were 4 and 2 and she was having parties around them constantly, alot of alcohol etc around them, very drunk herself and them describing her vomiting after drinking and telling us that strangers were coming into their bedroom whilst these parties took place to "give them money".

We had the kids living with us for 14 months whilst it went through court and she did sort herself out a bit over that time so when we had the final hearing (2011) she and my husband were given 3.5 days each per week. Things went OK for awhile but then we started to notice more and more issues. The kids weren't dressed appropriately (in clothes way too small for them, but when we offered to provide clothes she refused, telling the kids the things we bought were "trampy"), the kids were coming home hungry saying she hadn't fed them, but she would refuse the shopping we sent with them. (She had enough money as she was claiming tax credits and child benefit for one of the kids and we were claiming for the other). There were many boyfriends in and out of the picture, being introduced and moved in within the same weekend and at times there was a different bloke living there literally by the next weekend)and the kids were describing them using drugs. Throughout all this we attempted to work with her to improve things. Social services were involved on and off with her throughout all this time. Things would improve, they would sign off and then a few weeks later something else would happen and they would have to be involved again. Eventually things got so bad that my husband had to take full custody again. The court case went on for about 20 months and the kids lived with us and had supervised contact with her at our house each week throughout that time (as when we tried to allow daytime contact on Xmas day, she abducted them, refusing to tell us where they were until the next day). Gradually she was allowed to start having the kids on her own for a few hours or overnight, but she wouldn't ever take them to her own house, it was always that they stayed at a boyfriends or a mates with the kids sleeping on sofas rather than the beds she had at her house. In 2017 my husband was given full residency and she was given a contact order for Thursday and Friday night one week and Thurs, Fri and Sat night the following week. At that point we were also awarded child benefit and tax credit for both of the children as we were the ones providing clothes, food, school trips etc for them but we continued to offer food and clothes if they were needed. At that point she was claiming for herself and a younger child that she had had to someone else. We thought things were OK for a short while but then my stepson behaviour began to change drastically. Prior to this point he had always had a very close relationship with his father and myself.