Most of my theories have Misty being 100% aware. If Misty killed Haleigh at or near the MH, and Tommy somehow ended up involved with it where he is the one who carried Haleigh away, then she might not know exactly where Haleigh ended up, but other than that, I think Misty knows everything else.
As far as Tommy, I haven't researched extensively into his background, but my impression is a downward spiral after Haleigh was gone.
MOO, I don't think any of the people in this case are completely 100% selfish and without feelings. I don't think there are very many, if any, people in the world who consider themselves 'the bad guy'. IMO, most people have rationalizations and justifications for their behavior that may not hold up to almost everyone else, but this is the belief system they operate under, and I think understanding that goes a long way towards understanding actions that otherwise seem so inexplicable.
This is why I can see the possibility of Tommy helping his sister because he loves her and wants to protect her, especially in the moment, but after it's all over, struggling with what has happened. If this is the case, it wouldn't surprise me if he talked to reduce his sentence, and it also wouldn't surprise me if he talked because he just couldn't keep quiet about it anymore.