I am one of the posters WWIFN spoke of (I think. I was in your position 3 years ago.
My precious, precious DD2 is now 2 and a half. The OW's DD is 5 months older.
I am not with my XH, though no lack of trying on my side. I tried and fought and cried and blamed and hated the OW, for a long, long time. It almost destroyed me, there was no way in high hell to save my marriage, but I had to figure it out by myself. It almost consumed me with anger and bitterness and hatred. Be very careful what you are fighting for, and it will only work if your H is fighting too. By "helping" him and colluding against the OW, you are preventing him from taking any responsibility for his actions.
3 years down the line, I am a different person, changed by what my H did. We are now divorced, he is still a twat and always will be, I can deal with him much better now.
My DD's have a good relationship with their sister (we don't call her a half sister, she is their sister and they love her),I have no realtionship with her or the OW, but it is easier as I am no longer with their father.
But it was the hardest thing I have ever done, to put aside my hatred for the OW and her child, and now I couldn't care less about the OW, and my feelings for her DD, are based on my DD's affection for her.
It's been a long hard haul, don't underestimate how hard it is, good luck, I fear you will need it.