I can't say one way or the other if the future stepmother is a terrible person or not. On its face, her comment about excluding the younger sister was mean-spirited, but again we have no idea if she meant it due to her own vanity or whether the sister's behavior towards the stepmother caused some bad blood. Is 'daddy's girl' planning on making the wedding all about her in some way? We just. don't. know.

OP's comments about whether he should "let [stepmother] make a fool of herself" tells me there's some underlying animosity here too. If everyone really was as happy about the dad's relationship as previously stated, I would expect more confusion or questioning of Maria's words, not the immediate desire to break them up. It sounds very much as if Maria is not that welcome as an addition to the family. Is this because she's a Disney villain or because the kids don't accept her & have let her know in not-so-subtle ways? Again, we just don't know, so I hope everyone will excuse me if I'm not ready to tar & feather her quite yet.