Caren, I'm sure a lot of GPs who are in this position (or worse)never thought they would be there. (((Hugs)))
I agree with PPs (previous posters) that this resentment of DIL's may have been building up for years. In that case, of course, it was a little unfair of her to keep turning to you for childcare and so forth. But, TBF (to be fair), maybe she kept thinking things would get better.
Did she ever indicate to you that she had a problem with something? Or did DS? Maybe if it wasn't resolved after all these years, they decided that some space was needed?
Or perhaps the issue arose more recently. Have you or DH had any trouble accepting that your GDs don't need you as much now (though, of course, they still love you)? It's good that you don't "nag" and so on. But is there any chance that you or DH tend to upstage the parents at the games (or try to) in any way or that they feel you do? If so, this may be another reason why they feel the need to distance you a bit.
The fact that the word "enmeshed" was used concerns me. Does DS turn to you for help that DIL feels they don't need (financial or otherwise)? Perhaps this has become more pronounced in her eyes now that the girls are older and you're not as needed to help with them?
Anyhow, I'm glad you've backed off and I hope you continue in that vein. Enjoy birthdays and holidays if invited (sounds like you will be) and please resist the temptation to use them as an opprotunity to say, "wish we could see you more often" or even "miss you so much." Stay light-hearted at these events and in any cards and so forth. Hopefully, in time, DS and DIL will feel more comfortable with you again and the visits will increase.