Heck this is a difficult situation.

I think you really need to talk about this with your fiancee. Is he going to be "on your side" and at least keep peace with you and with his mom... or is he always going to be "in the middle" or worse, take her side. Even if he feels torn between the two of you is he going to be able to keep himself from being overstressed?

Why did he build the house so close to his mother? Did she insist? Does he need to be close to her? That isn't necessarily bad, but if there is going to be constant conflict between the two of you, putting a little distance would probably help somewhat.

If you really love him, and since you have already made a large move to be with him, this is something really worth exploring. Talk with him about it... does he think this problem is going to "magically" go away or does he recognize it and plan to work HARD to make it work. It may require family counciling. I don't think it is necessarily the case that he will have to choose between the two of you, but he is definitely going to have to make some difficult decisions that are going to seem like making that choice (and very likely his mother will emphasize that). For example if it is necessary for your family to find a new home some distance away from his mother, that doesn't mean he has to cut her out of his life, but she may make it seem so to try to force his decision.

I don't think anyone can give you an answer on this. People here can give you some advice from experience (I am not married and have no children... so I am only commenting on what I can imagine) but you are right to be concerned and to make this a serious consideration when considering your choices here. You have already put a lot into this relationship but if it is going to cause ongoing trouble then there is no reason that you can't change your mind if that is necessary.

Consider the environment for your daughters as well if there is going to be constant stress in the household over this.

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