I know what you are going through and it is tough. The first thing you must do is think long and hard about what you are taking on. Children will tie the relationship to their father forever. There is no escaping this. You will never become their dad even if you become their step father and you must give this to them in your heart and verbally, never stepping between them. You must put yourself into his shoes with the feelings of all that he has lost.
If you accept this, step two is good healthy open communication between you, her, and each child. Keeping the communication open in a supportive way will let them know that you are not the enemy. Supporting them the way a good dad should will be seen by them and they will migrate toward it.
You can be their protector and only step between dad and child to stop physical abuse.
The only way you will reduce his influence is to move a good distance away, if you marry.
Lastly remember, the fight between him and her is not your fight. Keep yourself out of it as much as possible even though you will have to provide a listening ear and a crying shoulder. You also must tell her that you are not her warrior, you cannot fight her battles, but you will referee if things get out of hand and support her decisions in this matter.
Good luck, once again, we know it is tough.
Advice- She needs to get a restraining order against him, or hopefully have him arrested for shoving her or hitting her. She needs to have a record of it. (No record, and it won't matter what she says in court.) Ideally get the guy arrested at least twice for domestic violence or disputes and have a restraining order in affect.
THEN she should file for child support.
He will get supervised visitation only at a center he has pay to visit every other week for 2 hours. This will go on for a few years. He will also need to complete an anger management class.
Then he will get every other weekend assuming there are no other incidents.
With this type of custody arrangement, the father will have very little influence over the children and you should be fine. The kids will believe whatever you and mom tell them.
A few more things you should know- If they were married and she is not allowing him to see the children, she may lose custody of them. She needs to get her ducks in a row to prove to domestic violence as quickly as possible. Also, if they were not married- She should take the children, move to another state or country, and then file against the father there. He will need to follow her to where she has moved to if he wants more than 2 weeks visitation in the summer. If they were married this is kidnapping, she should not move until this goes through the courts.