My wife does it at home, and it doesn’t bother me. If she did it in public, it certainly would.
It’s disgusting behavior in public. Why? Because my mother said so.
Well, if it’s a stealth “unbutton” in public it’s nobody’s business.
If you go full Al Bundy, pants wide open, hand down pants with a loud " OH MY GOD I’M SO FULL". I’d call that unacceptable.
If you can eat a meal, and not have your clothes fit anymore, then your clothes don’t really fit. Buy one size up.
Sounds like you either have body image issues, or food issues, IMHO.
No. Never. If the button is undone then the zipper follows suit all itself.
Invest in stretchy pants.
Sounds like it’s something specific to your body and how it works and it’s a ‘don’t comment till you walk a mile in my shoes’ issue. You should explain that to mommy.
If I was your mommy you wouldn’t do that in my presence, buy bigger stretchy pants and teach yourself to eat and chew slowly That just sounds rude, and I cannot imagine a place where it wouldn’t be rude! Eat by yourself alot??
I highly recommend that you unbutton your pants before you start eating and then say AHHHH! after every sip of your drink, fart at random times (with one side of your butt cheek lifted) and talk loudly with a mouth full of food throughout the meal. I can guarantee that no one will notice your pants. Problem solved. At the end of the meal say, “Thanks for the grub. I feel 10 months pregnant with food baby octopulets now”.
Whatever you do in your own home is up to you. If your mom would prefer you not do something in her home, then out of respect you should not do it. Eat less or wear stretchy pants.
It seems like there is something wrong with you physically that you bloat up that much every time you have a meal. It’s never happened to me. Have you talked to your doctor about it?
As long as your shirt is long enough to cover the button, it’s perfectly ok.
I’ve been known to covertly loosen my belt a notch after a large meal, so I can understand the concept. (My pants have a little slack in them - okay a lot of slack, since I’ve lost some weight. I’m a guy so I can get away with it.)
But unbuttoning my pants? Oh the hells no. Not even if they were uncomfortably tight. And if some charming lady started disheveling her clothing around me I’d be pretty weirded out.
It depends on which “button” we are talking about here.
You could have foods allergies if you keep getting bloated after eating , if you doing it at home I see nothing wrong with unbuttoning your pants button. But I would try to if get bloated when eating certain foods .
At home? I live alone, so there’s no one around to comment. Eating out, or at someone else’s house? No way! My pants are about one size looser in the waist, so I have some extra room, anyway, in case I eat like a boa constrictor after a long fast.
I think you should get some pants or skirts with elastic waists, so you can be comfortable. I would be quite surprised if I saw someone unbutton their pants at the table.
I can honestly say I’ve never done it or wanted to. Maybe I should wear tighter clothing?
Now that I have done; both home and in public. 
In public? Yes, it’s taboo! In the privacy of your home, do whatever the hell you want. But out in public, or in front of guests, for the love of god, please don’t! Your pants either don’t fit properly, or you’re eating way too much.
IMO its not as bad as picking one’s nose or farting at the table though. I’d let it slide in most cases, unless I didn’t like the person, but that is more of an excuse to be a dick to them. Its not social taboo, like using the water fountain at Target as a bidet. I see it more akin as to adjusting your pants or unzipping a jacket or something. Just leave the zipper alone on the pants and it’s all good.
I don’t know why this comment triggered it, but I remember some stand up routine from many many years ago (probably in the 90’s). The comedian was saying that he loved the new button up fly. He could “undo one button for dinner, one for dessert and one for Baywatch”.
You and the OP are siblings?