i love to explain my opinions so here's my actual drinking headcanons
siffrin - doesn't seem to get tipsy. he mostly just gets less anxious, which means he's less worried about saying the wrong thing but also less concerned with pushing himself to talk, so he's basically behaving the same way and just having different emotions about it! they appear perfectly stone-cold sober until they eventually suddenly hit drunk drunk and now they're liable to fall flat on their face and not remember it the next morning. takes a while to get there though, they may be small but they're also prone to alcoholism. (drinking to self-medicate mental illness is not a good idea, everyone.)
mirabelle - thinks beer smells nasty; wine's better, but grape juice is one of the worst fruit juices to begin with so eh. she'll have a hard cider on occasion or a fruity mixed drink, but only one and maybe not even the whole thing. she doesn't like feeling weird and she gets worried about acting weird!
isabeau - gets tipsy on half a mixed drink and then gets steadily more drunk from there. historically hasn't gone past two drinks in a row for fear of messing up his himbo persona, though he metabolizes it fast enough that if he's staying out late he can have another drink or two. being drunk turns isabeau into even more of a clown; he wants to have fun and he wants everyone else to have fun and more importantly he wants them to like him!! he'll fetch the next round, tip generously, share his food, join unwise schemes just because someone asked him to and pointing out the risks would make him seem like a square, etc.
odile - would drink vodka straight out of the bottle and get reckless as a youth. good thing cars don't exist! she no longer believes that she's too smart to make stupid decisions, but she still likes to cut loose with a couple shots, which she's very consistent about: she knows exactly how much alcohol will get her pleasantly buzzed without leading to a hangover. she's a lot more open and social when she's drunk. she's happy enough with her usual dry humor and too-old-for-this attitude, but sometimes it's nice to have an excuse to get uncharacteristically silly and dramatic for an evening instead!
pétronille: doesn't drink. she says it's because she has too many responsibilities, but she doesn't like to be around drunk people, either.
euphrasie: likes wine. she might have a glass or two to unwind after a long day, or pop open a nice vintage on a special occasion. she's willing to try other drinks, but she already knows what she likes the best and she's definitely not looking to get wasted. being too impaired makes it hard to do all the things she wants to do! she's more affectionate when she's tipsy, which is saying something because she's already quite enthusiastic and loving and touchy.
claude: prefers other substances, but isn't opposed to drinking. she's not picky about what the drink is, though she likes to try weird mixed drinks and euphrasie's wines and homemade brews. she doesn't see the point of having one or two drinks; if she's having more than a sip then she's getting blasted. she isn't a very inhibited person anyway, so she's not very different while drunk, just even louder and more distractible.
loop: can't get drunk because they can't consume anything. if they could, and weren't pulled into the party by siffrin immediately post-canon, they'd be physically dependent on alcohol within a month.