Even as a non-custodial parent, you do have rights to keep your children safe and healthy. If you know that the stepmother is being abusive, you need to start documenting it. If you feel that the children still need counseling, or are not getting proper medical care, you need to request that your ex provides that. If he won't, then there is probably recourse written into your custody order - bring him to mediation. Bring him to court.
You are never going to change your ex's feelings towards you. You can only do your best to parent the kids within the time that you have and using the legal recourse that you have.