Kathy, it looks a little fuzzy. Could be that they have entered into a co-habitation contract, which more or less equals a regular marriage, which of course is also a contract. I don't know the context but it could very well be that they living arrangements, with or without a contract have turned into something that would legally be considered as a contract. You often see mistresses suing monyed people to obtain this type of 'status', and sometimes they are successful. It looks from your context that the latter might be the case.

Just the fact that they made a will, seems to point to this too, and you are very right that this is legal.

Good luck,
Marijke