I would do my best to limit the contact between your stbx and both kids. What he is doing is abuse. You probably won't be able to get supervised visits. But you can minimize the separation between your kids.
>>>>>>>> What on Earth makes you advise people to choose THE MOST CONTENTIOUS METHOD to deal with every situation ?
The courts aren't going to separate siblings that young. It's not in their best interest.
>>>>>>>>> They are half-siblings .
-------------------- Never let your sense of morals get in the way of doing what's right. Isaac Asimov