to say no to sisters request for no alcohol over Xmas?

Perhaps if his sobriety is fairly fragile he would be better off spending Christmas with his own friends and family, and DSIS might prefer to join them and celebrate with her family on another date?

Could also be that he doesn’t actually want to come, has used presence of alcohol as an excuse and she’s trying to eliminate that barrier.

Prepare for him to get pissed and her to blame you OP.

suggest your sister might not be in the best relationship if these demands carry on
it is his issue
no need to make it yours, or even hers

YAnBU we don’t drink and host Christmas. We still get drinks in for everyone else who joins us because they do. It’s part of accommodating everyone.

Also, you’ve offered her an alternative and been willing to compromise if it really is important. They’ve declined that, that’s their choice but not your responsibility.

She's been with him a minutes and expects everyone to change things for him. If he's been sober for 3 years. He's been round people who've been drinking. Her request is unreasonable.

You’ve offered a good compromise, your sister can choose between joining for an alcohol free lunch and Boxing Day (sounds like that’s probably alcohol free) or come for the whole thing and accept there’s alcohol.

YANBU OP.

There are plenty of alcohol free alternatives wines and beers available now. It’s not your sister’s place to ask/demand a booze ban for everyone. If he is falling off the wagon she’ll find it exhausting trying to constantly watch him.

You're not being unreasonable, she is. Especially for arguing after you already told her no, when you, not her, are hosting.

Also, a boyfriend she's only had for five months is not "part of the family." Who knows if he'll even still be around by Christmas.

But you've offered a compromise anyway, to refrain from serving drinks until after dinner, which is gracious enough. I'd tell her no one more time and tell her to let you know if they're still coming.

I also jumped immediately to the thought that he’s not quite so assuredly sober as she’s telling you, and she doesn’t want him to be on the booze in front of you so is trying to control the situation. It would make me worry about her tbh.

But has he said any of this?

I've been sober for 4 years, probably would be considered to have had a problem with alcohol previously, and no way would I put demands like that on anyone let alone a family at Christmas. After a few years you are strong enough in your sobriety to be able to be around people drinking alcohol and if you're not, you avoid those occasions. His sobriety is his responsibility, not yours. And I say that knowing a lot of people in recovery and having been there myself. YANBU

This post is spot on. Part of most alcoholics recovery is learning how to be in situations where other people are drinking and finding your own response to this. Now it could be that his response is to remove himself from that situation, which is fine, and your sister is wanting him around for the whole thing. So then a compromise will need to be found and your offer of a part of the celebration without alcohol is a very good one..
If he was a long term partner and early in his recovery journey I'd probs offer a longer/more compromises but as this is a short relationship it is really big ask from your sister. Perhaps if you could compare it to if he was vegan and had a real issue with the presence of meat and therefore asked for the entire period to be meat free. So offer the problem back to dsis which is that you as a family would like to enjoy some alcohol over the period from Xmas Eve to Boxing day so can she please discuss with her partner how they can get this to work from their perspective and come back with a reasonable alternative between the two extremes of "no alcohol at all" to "alcohol served at every possible opportunity "

It does sound as though his sobriety is not solid. Asking others not to drink at Christmas is fucking rude!
Especially as they haven’t even been together for six months yet.
Like others, I bet he has fallen off the wagon and she is not happy.

I know there's a lot of variation/everyone reacts somewhat differently, but it really surprises me that someone who's been sober for three full years cannot handle occasionally seeing others drink alcohol in the context of activities which aren't even alcohol-centred. I would suspect that this is not his request but your sister's and that she either misunderstands the situation (he CAN handle seeing others drink) or has mischaracterised it to you (he hasn't really been consistently sober for 3 years). If she persists, I'd consider calling her bluff and asking to speak directly with him about how to minimise his discomfort without massively disrupting the household or celebrations.

It may be that there's something particularly triggering for him about Christmas and he feels he can't cope but IF he is genuinely afraid he will abandon three years of sobriety and drink at Christmas if he sees others drink then he probably should be spending the holiday in surroundings where he has tight control and knows exactly what to suspect, not spending three days in someone else's house with people he barely knows.

She is being unreasonable, I presume you have checked with everyone else that they are happy for an alcohol free meal as I am not sure why this short term partner should be accommodated at everyone else’s expense.

There isn't a part of this I would accommodate, not even Christmas day lunch.

The best part of Christmas for me (now kids are much older) is champagne flowing all day long.

It is that single time of the year where alcohol and chocolate at breakfast time is not only perfectly acceptable, but positively encouraged.

Your sister is entirely unreasonable and it would be a hard no from me. If he can't be around alcohol then they should stay home. I wonder if she makes such requests at restaurants 🙄

if it was your sister and she was a recovering alcoholic and recovery was fragile then YABU

But for a man she’s only been with since May? Nah!

If it were a family member very recently trying to stop an addiction, I'd be sympathetic. Its a hard road.

But she's been dating him since May, and he's "3 years" sober, then no. So either he's not actually sober, in which case I'd suggest she has bigger issues to consider than Christmas, or he is sober, but actually I wonder if she's trying to also be sober and its her that would find it difficult. Either way, not your problem.

Yanbu. I never understand demands like this. He can't (or she can't to!) expect everyone not to drink just because he's got issues.

And if he's still a problem drunk, making everyone else stay sober won't stop him drinking. Addiction finds a way.

What nonsense. He’s a recovering alcoholic. That doesn’t mean every other person he associates with is not allowed to drink and if he feels that uncomfortable being surrounded by everyone having a drink then he shouldn’t come.

She's been with him for only 6 months and is now calling the shots. Tell her to jog on.

You are not being unreasonable at all, anyone abstaining from alcohol needs to have the courage and willingness to not consume alcohol and their partner needs to trust that they have the will power and intestinal fortitude to stay alcohol free.

He is not as sober as she is letting on.