This family sound like wrong uns, so to speak, if I were you I wouldnt even want my kids seeing any of them. Sounds like you do the best for your kids and then they destroy all your hard work of trying to provide a stable home and life!
I have been in a similar situation, and I decided to move away, I dont get any maintenance money and quite frankly, I dont even want it, I am more than capable of giving my son everything he needs, and if I were to take maintenance it would be a foot in the door to my ex to see his son, I would not let this happen as he has clearly used my son in the past as a way of trying to get back with me! Sick!
I think the best thing for you to do is to keep them all at arms length, dont respond to any nasty texts or any accusations, thats what they want! Keep out of there game and they wont have anyone to play with, they will get bored of it in the end, trust me! I even took the step of moving, it helped a lot!
Cant you get an injunction on him? He doesnt sound like a very good role model for your children!