ODD, self-harm & consistency

I have 3 teenagers. One learns & processes things differently, and is very willful against anything he hasn't chosen so likely ODD. We started TSG about 3 months ago and have been very gracious with all of them. They are all making progress, however we had a huge breakdown Monday and I had to excuse myself to calm down.

The one son has had thoughts that he doesn't fit in the family and should leave as we be better without him because he's so different – either run away or suicide. I've had family and frienda who have dealt with suicide in the past and it makes me very worried. Since he is ODD, he doesn't take no answers and then just hates me. Ive been working as you suggested in a video to get him to see he's fighting his own choices, not me. We are making progress. He used to walk away or go totally out of control yelling and hiding in his room. If I make him comply competely he'll continue to power struggle until he usually ends up out of instructional control. So now, when I can see him comply with a direction partially now, I don't give him the consequence. However, this is now causing problems with my daughter, 2yrs older, because she feels in favoring my son and I should be very strict and give him full major maintenance etc because he's not following directions. She doesn't like having to wait through power struggles which are mostly during Bible study. I don't want him to leave (what he wants) so try to get him to choose to join.

I'm afraid bring too strict when he's not really agreeing with or understanding TSG might drive him to self-harm in some way, but my grace is causing my daughter to feel she's second best and not heard and she doesn't want to have to keep dealing with his interruptions.

So the question is, how strict should I be without driving the one son to self-harm and how can I help my daughter not feel so hurt by the situation? (She undermined me in front of everyone which was out of role, telling me I was wrong and then was nearly in tears with the injustice of it all.)
Many thanks!