You are marrying him but you are also becoming a step-mom if they are sharing your home even just on weekends. With the problems w/ the ex wife you are in for a bumpy ride and I suspect that he could probably get primary physical custody of his children with his wife having supervised visitation. Keep in mind that he will always have a connection and have to deal with his ex-wife because of the children and ask yourself if you are prepared for this.
He will need to file a motion for modification of custody or a complaint. He can list the issues which you have mentioned, especially the fact that DSS has been contacted and the children have been in jeopardy. He should document all conversations and e-mails he has with her and also any contact he has with his children. He should contact JAG for advice immediately.
You may only be dating, but a year with the same person in this day and age does count for something. The court should determine the merits of him getting custody based on him providing a stable environment for the children. However, your relationship can also be used as an argument for a stable environment, especially with the plans of purchasing a home. You may be just what those kids need. I know my children have told my Fiancee that sometimes they think she loves them more than their own mom.
Having been in a similar situation with my husband while he and his ex went through this I can tell you this: My husband’s ex was asked on the stand in court how many men she had been sexually active with since they had split up and the answer was 4. She was asked how many of those men the children had met and knew as “mom’s boyfriend” and she lied. They also asked my husband the same question and I was the only one. In my opinion it shows enough about your fiancee’s stability that he’s with you and has serious future plans. And you may be called as a witness also. The court can look into your background or anyone else that may have close contact with the children. They do this for the best interest of the children, so keep that in mind if the questions seem to start getting too personal.
As far as getting married right away, only if that what is right for the two of you. It may make a difference but make sure that you do it for the right reasons…I mean, what if he doesn’t get custody? Welcome to the club, btw, it’s an adventure being a step-parent.
Good luck to you!