I see something of both. It's doing the OP a disservice to deny the former, because if she continues in the same vein she is at real risk of doing irreperable damage to her relationship with her DS.
Yes, some of the posts upthread are too much. It's a spectator sport around here to kick people who are already down. But I think it important to differentiate between this form of cruelty and some of the wise if blunt advice given on this thread.
OP has posted here in an attempt to understand and to justify her position and in doing so has received a lot of advice she clearly did not want to hear. As it happens, what she wanted to hear wasn't what she needed to hear. On this occasion at least, MN has been cruel to be kind. That is what this site is for.
Hurt people, hurt people. This impasse is where insurmountable family divisions begin. I appreciate that the OP is hurt. And she will be more hurt in the future, should her DS and future DiL remove themselves at a safe distance from further behaviour of this sort.
FWIW I have no skin in the game. I find the very idea of an 'engagement party' anathema. I can't abide identikit formality, overplanned lack of sponteneity, and dismal, tedious speeches even at weddings, let alone the myriad satellite events people are expected to attend in addition.
That matters nothing. It isn't about what I think, and it isn't about what OP thinks, either. As a loving parent it's hard to let go of the apron strings, but her adult son is no longer a child she can tell what to do. Her choices are simple: either recognise this, or be prepared for a lot of unhappiness in the future.