It sounds to me like you are de-flatting the potential long term drama by asking that your son be returned to you and his home.
Your brother in law is probably blinded and with tunnel vision and though of wanting to help his sister and does not see he is only acting in her best interest not yours or your sons.
At this point in time I would not trust either.
I would also strongly think about a legal separation where child custody/visitation is made iron clad. Don’t wait for them to make that move first, you do it.