Max1465. What a difficult situation to be in, with so many problems.
Oddly enough I think the problem between your two children is the least of them. This does not sound to me as if there is a real estrangement between them, more that your older daughter, pregnant herself, is overwhelmed by her younger sister's problems and her refusal to do what is obviously necessary to all but the victim, separating from her abuser. She wants to protect her unborn child from the stress in the family so is choosing to withdraw and just not speak to her sister.
When you younger daughter breaks from her addiction to this man, I think you will find the problems between the sisters will gradually resolve.
I would suggest that for now you accept that your 2 daughters are not talking, but sit down with the elder girl and tell her this and tell her also that with Christmas coming you want to be able to see her but you also want to be able to see the two little ones, who are the completely innocent victims of this - and that you want to help your younger daughter ditch her partner finally. Ask your older daughter for advice and help and suggestions about how to deal with the arrangements for Christmas, I think that if you do this, while your 2 daughters themselves may still not speak, you can between you work out a solution that is stisfactory to all.
If your older daughter is a bit resistant at first, let the subject lie and then gently bring it up again a few days later. I am sure if she feels you want to work with her and accept how she feels about her sister you will end up with Christmas. at least, that workd for all.