Ah. I see. You're not into replacing the BM. Got that. And the kids are going back to her full-time anyway.
So here's my final words of advice to ensure the solidarity of your marriage:
Remember not to make him see YOU on a different side from HIS KIDS. His kids are a part of him. If you really are the love of his life, you will love all parts of him and that includes his kids. Otherwise, you're rejecting a big part of him. And he will feel that even if subconsciously.
You may not want to replace BM, but you need to love them like a stepmom. There are a lot of wonderful stepmoms whom the kids end up loving and respecting even more than their BMs.
Your husband will love you even more when you can love his kids as much as you love your own. If you plan on having kids with him, be very very sure not to show that you play favorites.
You're young and it might take some time for you to understand that this is humanly possible. Don't take that offensively, but I've learned that when we're young, we somehow feel that we can only love so much or that our love is territorial. We pick and choose who we will love more, etc.
At my age, I've learned that you can never lose by being loving. You are capable of generating endless love. For everyone.
The more love you give, the more you receive. From everyone.
Good luck, darling.