Motivations... Everyone hates this movie except my entire family.
I find it interesting reading WHY people have such passion against this movie. My family is very similar in so many ways to the Stone family: tight circle, laid back and liberal yet also overly judgmental if you are different, and quick to talk behind your back. Not to this extreme, but these character traits needed to be amped up in the movie because they only have two hours and with all those characters, there was no time to be subtle or to develop slowly. It's a movie, ya know? I also found this family to have similarities to A LOT of modern-day Americans. It's a cynical and catty world we live in today.
Motivations...
I think haters are too quick to disregard the effects that Sybil's ongoing health status was supposed to have had on the characters. Before they even find out how bad it is this last time, they had already been affected by past battles. It is not overtly discussed or made clear to the audience because that is not how they carry themselves.
I myself once had cancer (the year before this movie), my family still joked viciously, and we never lost a beat with being snarky.
Sybil wanted an unrealistic perfect girl for Everett before she passed.
Everett wanted to hurry and get married either to fix mom or, more likely, just to give her that eldest son's marriage before she died. (this happens a lot with families)
Amy was Amy and I know A LOT of them.
Kelly was worn down. The dinner table showed it. He did feel bad about it after.
(they all seemed to acknowledge their behavior during different points in the story)
About the dinner...absolutely the Stones were sensitive and quick to spew venom at people that didn't accept Thad and his life. Families that have lived with, and are used to, atypical situations within their family are often quick to set-up questioning people JUST so they can have that fight. It's unfortunate and ugly but many people are often overly-defensive about such things and are quick to fight outsiders.
The Stones are flawed, catty, very passive aggressive, and sometimes plainly mean. Still, they are character examples of many more of your neighbors than most of you want to accept. Harsh and cold on the outside but actually quite warm when they take their heads out of their own asses. Just like my family.