Well, obviously I will never be a mother in law. However I have two daughters - my late wife's first by her husband, and a home-brew. I also have a son - my late wife's, by her husband. And when I say "wife" I don't mean "awful deadlock" (sorry, that should read "lawful wedlock"). And I am in the UK not the USA.

All of these things may affect the issue of proprietorship - which is the wrong way to look at it.

I hope that my three children know that when the chips are down, right or wrong, if they battle with their significant others I am with them.

But the OP is going to another issue.

I fear it may be too late. If you want to be in a good relationship with your child's significant other (that other being of the same gender as you) you need to make that friendship from an early stage. You need (while being careful not to undermine your sprog) to be standing by the other while the other may be joshing your sprog. You need to be finding common ground with the other - hifi, cars, guitars (that's mostly man to man, although I know some female petrol heads and plankspankers - and audiophiles) - or whatever women bond about. Then you have a child, and as well as that child, a friend.

I am I think a pretty good mate with sprog's significant other - we can do and (unless he is a better actor than I think he is) both enjoy digitising my ancient 45s together, or planning uses for percussion, or having a beer while sprog brands us both nutmagnets - and I am still friendly with my older daughter's previous very significant other even though he and she split on not good terms: if he crossed the line they both know where I would be but he is my friend now, not just her ex.

I that is where you can get to be then there is no war over your child's attention.