Welcome, Lolo!
I want to add to what Smileless has said that sometimes a parent's relationship w/ their AC may seem hopelessly strained/estranged, so they focus on possibly being able to retain/regain access to their GC.
As for the OP's situation, people, I imagine it's very hard for a parent to see their beloved AC at odds w/ each other, especially if it gets to the point where one is banning the other from her wedding. I imagine it would also be very painful for that parent to contemplate attending that wedding, knowing their other AC has been banned from it. And I agree w/ Smileless that most parents/mums would try to diffuse such a situation.
However, I'm not sure that "reasoning with" ODD means the OP was simply trying to diffuse the scenario. I suspect she tried to convince ODD to have YDD at her wedding after all. Very understandable, IMO, but a huge mistake. What she could apologize for is, as I believe I said earlier, "getting in the middle" between her 2 adult DDs.
I realize now that there may have been more that went on. Depending on how serious ODD was when she said she wouldn't mind if the OP didn't come either, she and SIL may have been angry that OP showed up, despite that. Again, I totally get the OP's decision, who would want to miss their DD's wedding? But ODD and SIL may have been shocked and angry that she came.
We don't know, of course, b/c OP didn't say. She did tell us it was "the worst day." I wonder if that's b/c there was tension between her and ODD/SIL. But again, we don't know.
Regardless, none of that justifies their using OP as a childminder during their honeymoon and then cutting her off for 3 years. If they couldn't make other arrangements, as one poster suggested, then, IMO, they should have found a way to maintain the relationship afterwards, not just used her and thrown her away.
Then again, we don't know if there were any issues that arose in relation to the babysitting or if there was a new confrontation after ODD and SIL came home. The OP hasn't answered my question about that as yet. Like agnurse, I feel as if there are pieces missing to this puzzle.
I also feel as if, sadly, ODD may just be using the OP again now, to watch her children even though she and SIL don't seem to want a relationship w/ OP, themselves (sigh). They probably feel OP wants access to the GC, and, no doubt, she does. But they're getting plenty of childfree breaks out of it,