What a complicated, interesting dream, thank you for sharing it!

If it were my dream I'd think I was trying to give birth to an idea or some aspect of myself. I think that your mother was across from you (and gave birth to you, now you are giving birth to. . .) and turned into your sibling (and you are all women) is a progression sort of thing, your mother got you and your sister as far as she can and now it's your turn to "give birth" to "Yourself".

I found the furniture arrangement interesting too, in that it was like your furniture was arranged a couple years ago and then you looked up and your sister was there instead of your mother. Did you have anything happen a couple years ago with the three of you? I use to dream about an apartment building I lived in the first time I saw my therapist, and when I saw her again 9 years later, I'd dream about it often and she said it had to do with what had happened between us back at that time. So maybe what happened back when the furniture was arranged that way, were you rebellious or your sister doing something you are doing now (or are you doing something now that your sister was doing then? Emotionally/psychologically, of course).

The plastic doll baby and your mother and sister being part of it; were it my dream I'd be thinking I am not as mature as my mother, can't give birth to a "real" child yet, am not grown up enough but am only like/even with my sister; not "ahead" and grown up yet. That it felt like menstrual cramps instead of childbirth pains and that your "stomach" was changing, that's what children think, that babies are in their mother's stomachs.

Are you in competition with your sister about something? Getting attention or more responsibility or respect, etc.?