Your difficulty at this point in time is that

you don't know, or are not clear, in your mind what role you are to play as relation to this couple.

You are still her mother. Work on this relation.

As to the other girl, she makes your daughter happy. Isn't that what you want to see, your daughter being happy?

Your daughter doesn't see this world as you do. You got to make room for that reality.

Your sense of loss is profound.

The loss you are experiencing is your sense of tradition, in addition to the daughter you thought you knew.

May the sympathy and empathy shown in this forum offer some solace.