Hi OP,
Firstly, I would like to wish both you and your Dsis heathy babies.
It’s a very difficult situation and although your mum should be supporting your sister throughout this, this must also be hard for her as she sees her teen daughter become a mum.
At this stage, I think you have to think of all possible Avenues to help secure your sister accommodation. It’s probably too late to do it before the baby is born. I also understand you not wanting her to be in a unit.
You said you don’t mind her staying with you as she finds her way into Motherhood. However, what if your mum doesn’t take her back after the baby is born?
Are you willing to have her for a bit longer and this could mean you might not be able to use your nursery for a sometime.
My daughter is 3 and she sleeps in my bedroom from the day she was born. This has amazing but I know that some parents need to have that nursery for things such as sleep training etc. Are you have to give it up?
Being a parent for the first time can be a massive shock irrespective of age. It can be harder for those who have not been exposed to it. For example, my mother became a mum at 19, however, she had loads of experience as she helped look after her elder brothers and sister children-by that I mean really involved in looking after her nieces and nephews as the parents when to work from a young age. In our culture, we like together with extended family etc( I think you get my point).
There is an 11 year gap between you and her, do you remember what it was like when she was a baby and were you a bit involved in helping your mum look after her? If so, and if you haven’t done it yet, maybe talk to her about the realities.
If you don’t have much experience, maybe you would discuss this with her. I know she is anxious so don’t know if this might make it worse.
This is such a difficult situation but please believe I am not judging your sister- not my place.
I fell bad for all involved and hope you can find a solution.
Could you have a heart to heart with mum and see what she has to say about her maybe moving back to set up before baby is born?
It seems that there might be some strain in the relationship between your mum and sis.
Hope you will update with some good news. 