baby daddy having another child.

Once I left my DBD he went on a crazy bender, partying and different girls for months on end. During this time he stayed with a girl for a few months and she ended up pregnant. My ex and I have 2 kids together, aged 7 and 5, and the girl is almost due to have her baby. He doesn’t want our kids to know because he wants nothing to do with the baby. Do I follow his wishes and keep it from our kids or do I let them know once their sibling is here? They would love knowing they have a sibling, but I’m also not 100% sure if the mother of that child would want them to have a relationship, although I know she loves my ex and is wants to have him in the baby’s life. I know it isn’t my business and it should be upto my ex to decide, but since we live in a very small rural town I don’t think it would be nice for our kids to grow up not knowing you have half siblings that share the same blood. Thanks for any and every opinion x

P.s my ex is also not mature enough to think of the kids best interest. He is very manipulative and narcissistic, also has turned into an inconsistent/absent father since I broke up with him. So he is disowning the new baby for his own selfish reasons.