But mine started when OS was a Senior in High School and DIL was a year older and in College, so it threw a whole new dynamic in our situation. He still had house rules, curfew, needed to get homework done, etc. . . . . . . I was trying to keep him on track with finishing school and getting ready for college.
Like Pooh, our DIL came into our lives when DS was still in school. Early early college. I didn't need to be as involved as Pooh was, since he was out of high school, living away from home, etc. But he met her when he was not even 21 years old, had over 4 years of school left, which we were paying for entirely, and our parenting was not quite done yet. When he lived at home in the summers, he had to have a curfew, not because we were controlling him, but because we had to get up and go to work the next day, and it wasn't fair to keep us up all hours.
So he resented some of the restrictions -- probably shared that with her. So maybe she resented us for that. Too bad. Once he graduated and we were not supporting him, we had ZERO rules for him. Fine. He was not living with us, he could stay up all night, didn't bother us. We weren't paying for anything.
We included her on one family trip. She seemed to think it was a getaway for the two of them and that we were in the way. She didn't realize SHE was the add-on.
I realized later that because he was still so young and we were still paying for everything, we thought of him as still part of our family, and that it was nice of us to include her in our family trip. Maybe they thought of themselves as the primary unit. I think they disregarded the fact they were not self-sufficient, they just wanted to be alone and decided it was us who was in the way. Ooops!
Might things have gone better if he didn't meet her until he had worked a few years? Maybe. Because we are totally hands-off, non-meddling. But it is tough if they meet their true love during that transition stage - when they are still dependent on the parents and the parents do have reasonable rules - but they think of themselves completely as adults. (disregard that $$$$$ we shell out every year to the university, folks)
And as long as we were paying for everything, we got to set some rules. They didn't like it, but too bad. Now they are both graduated, have jobs, support themselves. We express no opinions, give no advice, do not drop in, do not meddle in the least. She still doesn't like us.