What replaces them? How do you deal with people not scrubbing the toilet? Who decides who gets to live with who? How do you prevent abuse and over-dependency? What to do about love triangles?
I don't think these things are necessarily what defines the nuclear family.
Nevertheless, I think the answer to your question is relatively simple, albeit maybe not the one you want. As Marx and Engels describe, the nuclear family emerges not out of any intelligent design, but because it is what suits capitalism best (all of life is shaped to mold into the contours of the dominant economic system, as has always been before capitalism also). Similarly this will be the case in socialism. We may not be able to imagine what this will be like, but it is the economic system that sets the stage for what life will be able to be.
I don't think these things are necessarily what defines the nuclear family.
Nevertheless, I think the answer to your question is relatively simple, albeit maybe not the one you want. As Marx and Engels describe, the nuclear family emerges not out of any intelligent design, but because it is what suits capitalism best (all of life is shaped to mold into the contours of the dominant economic system, as has always been before capitalism also). Similarly this will be the case in socialism. We may not be able to imagine what this will be like, but it is the economic system that sets the stage for what life will be able to be.