If anything you're saying is remotely true then you should have left years ago. What difference does it make if your DH won't admit it is SD hurting BS. If you know she is, you need to remove your children from the situation. You let them be abused because you are more interested in your husband admitting it?

smh

Firstly you need to protect your son from her. He should never ever be alone with him.

Secondly, you can't make him parent his daughter. You can set up nanny cams and tape recorders and he will just get angry with you for showing him what a horrible child his daughter is. He will then come up with a million excuses as to why she's like that.
This is a world where what she doesn't isn't wrong. You pointing it out is wrong.

If things go really wrong, he will agree with you and blame it all on BM and try to get custody so that he and you can fix SD.

There is no miracle conversation or thing that you can do that will open his eyes and make him see SD like you do.

The best you can do is to remove yourself and your children when she is there and hope like he$$ that BM stays healthy and doesn't give up primary care.

I hate to be so cynical but if she has time to bruise and gouge him without anyone noticing then she has time to do a lot worse. I know it's a horrible thing to say but you are just as guilty as your DH for letting your baby get those marks. When will it be enough? When you catch her trying to smother him while everyone suppose to be asleep? No. You both are dragging ass at the cost of your other children. I would invest in cameras in every room so when CPS comes (and they definitely will) you have a defense for yourself or for your children she might also accuse of abuse. If I were you, I would set up cameras everywhere, I would find a place for my children to go on the weekends she is over and make it a regular thing, I would never be responsible for her alone or her necessities (bathing, cooking) so she will never have the opportunity to say I abused or poisoned her. I would stop pointing out the problems she's causing to your DH, I would ignore the girl's tantrums and outburst and I definitely would never fight with my DH in front of her because all of these things she loves. It's clear your husband will not hear it from you so you will have to wait until CPS, police, school councilor tells him she has problems. Until then stop falling into her traps and setting yourself up for fights.

You may think the relationship is worth fighting for but it seems that your partners opinion is somewhat different. He's not fighting for his relationship with You. He's fighting to make sure his relationship with his daughter is unchanged in any way. He is showing you by his words and actions that he will sacrifice All of you, the safety, well being and happiness of every other person he lives with to her happiness.

He accuses you of lying.
He vilifies your children.
He refuses to correct or discipline his daughter in anyway. Admissions of guilt are met with instant forgiveness.

You can to the nanny cam thing and present hard evidence that yes she really did Whatever. But don't expect that to change his behavior. He will still excuse her behavior. He will still excuse his non-reaction to her behavior.

The only way his behavior will change is if he gets some therapy for his guilt, but he obviously doesn't have a problem with how things are. When you come to him with a problem, he deflects and shuts you down and nothing changes. So he really has no issue. He doesn't have to punish princess and everyone else makes changes to let him continue on as he wants with no consequences.

STOOOPPPP IIIITTTTT.

Wow, I'm totally confused and now not sure who to trust here and who not. I know strong opinions can occur and honesty can be painful but wow, a little positivity and encouragement could go a long way. I don't have a sociopathic skid so I didn't have anything to add to original post but from the looks of neither did a lot of posts.

I'm not a psychologist or anything but it doesn't sound to me like your SD is a sociopath.

What is the visitation schedule with your SD? What is the visitation schedule with your three kids? It sounds to me like your SD had a lot of change in a short amount of time and went from being the "favorite" child with dad to being one of four and then one of five when the baby came. That is enough to make any 4 year old child act out in attention seeking ways.

I am in no way condoning any of your SD's behavior. It sounds like she could use some real boundaries, consequences and structure. I would say that if those things were all in place and she were still acting out then yes she may have something else going on but it really looks to me like she is a product of her environment and your DH's piss poor parenting.

I normally wouldn't advise anyone to give their partner an ultimatum but that is probably what it is going to take, "step up and parent or move on" just be sure to be prepared for both options.