What was the major event/mishap? It's ok if you really don't want to say, but we're all strangers here, and it might add helpful context.
A couple of major pieces of advice.
1) Consider the idea of your husband dating this woman separately from you. Just because you and he are a couple doesn't mean that you two should have to be a "package deal", or that another woman should be expected to be equally into you, emotionally or sexually, just because she's into him, even if that was everyone's expectation at the beginning. Similarly, if you and she had more chemistry it would be unfair to predicate your budding relationship on her and him clicking in the same way.
If it happens naturally great, but to require two relationships to blossom for either to continue just ignores the nature of human relationships in my opinion, namely that they ALWAYS develop in different ways at different times. Let it be what it Is and don't try to force anything. This site may have some helpful thoughts: www.morethantwo.com
. Especially the sections on jealousy and couple dating.
2) Whether you take my advice on #1 or not, under no circumstances should she move in with you right now. That's something that can MAYBE happen after jealousy and relationship configuration issues have been resolved and everyone Is feeling peaceful and groovy. Living with another person can be stressful, as anyone who's had roommates can attest. When that person is someone with whom you also have a confusing and sometimes painful relationship, on multiple levels... well, it will most likely ruin your ability to feel safe and comfortable at home, which is the one place where you should feel completely safe. It will likely be too much stress and make things fall apart fast.
I know the distance seems hard, but surely she has friends, and could find another lover who lives closer to her if she wanted, right (I assume you guys aren't requiring exclusivity within this new, long distance triad)? Wait at least a year... then, if everything is great, you can safely consider cohabitation.