I was very close to my mother as an adult, even though she was judgmental and critical. I think it was because she really was fun to spend time with and had a good sense of humor; I loved traveling with her as a teen and adult.

I have a tween who is very close to me (almost too close) and I think it's because I convey that I genuinely enjoy her company (especially when we travel), and am willing to do things she likes instead of forcing her into a mini-me mold. Fingers crossed that it will translate into a close relationship once she's an adult.

The "it backfired" part of PP's post is right on the money. Parents, even to adult children, still need to parent. The relationship is singular and a numerical age doesn't mean that the obligation ends or even changes, depending on the adult child.

Bad decisions, decisions that will harm their future (health, safety etc), should not be applauded or rewarded or ignored or financially supported without consequence. So doing, creates an enabling relationship that will harm everyone, perhaps most especially the adult child and also the sibling of the adult child who will be justifiably unhappy.