I don’t want my kids around their half sibling

My husband has a 9 year old with a woman who was a complete stranger to him until the child was born. This child has been through 3 schools and has been kicked off and banned from the school bus at each school, been suspended, and even kicked out of one school for being so violent to other children, teachers, bus drivers, and other school staff. His mother has been told he’s a danger to other children. He has been known to whip it out and urinate in classrooms or all over the halls and bathrooms at school and also his own house or he just goes in his pants so he can get sent home. She had him on ADHD meds for a while which made him even more volatile and kept him from sleeping and now has to have jaw surgery because he had started to grind his teeth so bad whilst they were figuring out his dosage. My husband has no parental rights or has even had a paternity test done. His mother uses the child to manipulate him for money and resources because my husband is so desperate to be a part of this child’s life. My husband is too afraid to fight for rights because she’s threatened to disappear with him. We have an 18 month old and a second one due very very soon. I have no intention of allowing my children to meet or have interaction with this child or open any doorway for him to have any “older brother” influence on them especially as they get older and this child has become more and more out of control with every passing year it seems. I fear this older child will resent my children for having two parents and a stable happy home and many advantages we wish we could give him too. My husband and his in laws are starting to see me as a hateful person and often try to guilt trip me into letting them have a relationship. Saying the poor child is just “disadvantaged”. They won’t babysit or allow him near their young children either but I’m expected to offer my babies up to this child whom nobody anywhere has any slight control over and who has severe behavioral issues. I know he’s just a child but I really feel like I have to protect my children. I’ll never lie to my children and they’ll always know he’s out there and I am all for them meeting and having a relationship when they’re mature adults and when my children are able to protect themselves and not allow him to mentally or emotionally abuse or manipulate them. I’ve been adamant and sticking to my guns but lately have been feeling very guilty. In need of advice.