I am so glad that you brought this up. It's not really anything I have thought of before as really being an issue for myself, and I am finding the responses to be very enlightening.
As for myself, I have a very severe sleeping disorder, and my bed is not just a piece of furniture, or a place for being intimate. My bedroom is where I go as a sort of sanctuary. Its where I suffer and also where I heal. At the moment my Boyfriend and I live in separate apartments because we found that to be easier for us, not just on a multiple partner level but on all levels. Even when we did live together, He had his own room and joined me on occasion or vice versa because of my need to be secluded when I sleep.
I really have no experience in this particular matter, but I do have a possible suggestion...
There has been alot of talk about the energy that the bed holds. I truly believe that all physical objects hold varying degree's of energy of the person that they belong to. A ring for example, or a childs favorite toy. You are clearly open to sharing your space with her which I think is very wonderful of you, but perhaps its the energy of the room, the furniture, all of it, that holds her back? I would invite her place something of herself in the room. A mirror, or candles or something along those lines to make her feel more at home. Then the energy of that room would belong to all of you and not just that of you and your husbands. It might be more comfortable for her that way.
Very much luck to you!