Lost in family drama

My SO has a half brother and sis that are 11 and 10 (he is 30). There mom is a drunk and never let them do anything or go anywhere. I would go into details about the rest of their home life but that can take forever. Anyway my SO try and spend time with his brother and sis. They share a dad and he died and recently his stepmom got a new boyfriend, moved him in and the guy been living off her. He is over the age of 50 and was living with his mom, no car, no home, no job not even a phone. And basically the step mom get disability for herself and live off the kids because they get checks as well. Anyway my SO been trying to get them and let them stay at our house just so they can be in a different environment and have more to do without her constantly yelling at them. We ask these kids what they want to do and go anywhere our treat and they can only come up with MacDonald they never even been to a movie. She let them come once they had a blast and I think she got jealous because from then on she never let them come again. They recently went on spring break they called and ask can they come over my SO told them let me talk to your mom. Apparently when he talk to their mom things where said about the guy living in the house that she hasn't even known that long and putting him before the kids. All this turned into we went to her house and tried to fight her and her boyfriend, at least that's what she told the rest of the family. They live 2 hours away, no one ever went to her house after she said no that was that because in the end they are your kids. Now my SO saying he not going over there any more period and he wash his hands with the situation he loves his brother and sis but he hate their mom and said he not about to keep putting up with her and he certainly not about to put up with a guy that has no relation to them. I feel sorry for the kids in all this. I have grown to love them like they was my own siblings and I know they adore their brother but who knows when we will see them again. Idk what to do because this not fare to them they just got caught in a cross fire. And they was looking forward to going to our son first bday party.