just continue smiling, say “may you be zoche to build a bayis neeman…” if new child say may you be zoche to raise him/her letorah, lechupa ulm”t… whatever just smile and show them you happy for them that’s all they care about.
about the kallah not dressed tznius, if your a girl saying to the kallah, matters on your relationship with her. look for something else like your hair/shaitel…makeup maybe just don’t comment on the gown itself