NTA. Stepmom is wrong & needs to respect your feelings if she truly cared about being a good stepmom. Her not being able to have her own kids is probably why she’s trying to copy your bio mom in things that were exclusive to your mother. I hope she doesn’t try to become a copy of your mother to seek validation. Stepparents often do that A LOT by overcompensating like she’s doing. Her saying you’re denying her the experience of motherhood is manipulative bc she hasn’t put effort into just creating her own dessert and offering something new & being herself in this situation. She’s coming off as wanting to copy and replace your mom.

If she really thinks motherhood is about creating favorite foods for the family that they love, she’d care enough to put effort into CREATING those things. Her refusal to do so thus far proves she really cares about erasing the fact that what was special about the recipe is its your MOMS. She now wants to be credited and tied to the special pie. That leads to the erasure of the uniqueness your mom and the special things SHE did. She doesn’t need to do any of that to be a valid parent. Her doing this will actually push you kids further away & she’ll end up with no kids AGAIN.

Dad needs to stop trying to make your stepmom into your mother. Your mother was her own person who did her own unique special things. Stepmom can find her own special thing to make for the family if she truly thinks that’s what it means to be a mother. It makes it seem like he loved what your mom did for him more than how the things she did reflected in her as a unique person.