Advice on co-parenting?

Background: I have a 4-year-old "step" daughter (I prefer to refer to her as just my daughter, but for technicalities' sake, she is not my biological daughter). My husband and his ex have co-parented for the entirety of her life - switching houses every Sunday. She's been living with my husband and I for 2 years, though I've been in her life since she was born.

Now, I'm pregnant with twins and my daughter is SO EXCITED to be a big sister. However, she's having trouble understanding that these babies stay at our house, even when she goes to Mommy's house. She argues that it's not fair that she have to go, but they get to stay with us. It breaks my heart that she feels different because of it, but we're trying to explain that:

1) Her mommy is HER mommy, not these babies' mom. She knows I'm not her biological mother.

2) Her mommy won't be a major, necessary part of these babies' lives, as she plays more of a role of an "auntie" or friend.

3) The reason she switches houses is because her mommy and daddy split, and it's important that a child have both parents.

4) Likewise, it's important that these babies have both parents, who in this case live in one house together.

5) The babies will be fed from my breasts and that her Mommy won't have the necessary tools to properly take care of the babies, as she doesn't have any other children - and, well, my daughter is four.

6) Overall, we all love her just as much as we always have, and these siblings of hers will love her as well (as she is their SISTER).

She's still having issues understanding, and argues that she has to take the babies to Mommy's house when it's her week. Regardless, her mother is not interested in involving herself in our twins' lives anyway, and I can't say I blame her. Any advice on how to make this process easier on her? I'm most terrified of changing any custody situations, as we miss her enough when she's gone and we would never want to take her from her mother. If, however, it's best for MY DAUGHTER, that's when I would consider it. I can't be selfish... Her health and well-being comes first and foremost, but I also don't want the little ones I'm welcoming into the world to be confused as to why their sister is never around. 🙁