Hi @Whitegrenache, how has this morning gone?
I can empathise, my brother is an addiction too. And our Mum enables him and likes us to play happy families. I also have 3 teens in the house (plus a tween). I think I may be several years ahead of you in dealing with my Brother tho.
My mum still gives us the "BD can't be on his own Christmas" line (even tho he has been for about 18 of the last 20 years). She gives is the same line every Sunday (because they also have a standing invitation to Sunday Roast every week.
I explain the above so you know there is no judgement from me. But what's important here is your boundaries with your mum. You should not have agreed this without considering your own children and partner first. That's putting your Mum above them - which doesn't make for a healthy family home, particularly when there is an addiction involved.
It took a good 10 years of reaffirming boundaries, but our rule is DB has an open invitation but only if he is sober. We've had many times of Mum pushing (or ignoring) that. Key is, DH and I are a team on it, we don't work against each other. If DB arrived drunk, we'd both be pissed off, rather than DH being pissed at me because my DB is an addict. So we would seeth together, talk to Mum as a Team, and remove drunk DB from the home together. DB has not been at our house drunk for over 10y now (he's been an alcoholic for 20y). He's only been here sober about 3-4 times in the last 3 years, so even that is dwindling.
If DP has his way, we'd have been NC with BD many years ago. But that would break my Mums heart, and that mattered to me, because my Mum matters too. So DH and I talked and compromised on the above arrangements.
The fact your teens are older doesn't feel like a factor in all of this. I've been in a bubble bath since 9.15am this morning, I can do that specifically because our children are older now. Not too old to all get up at 7am to open gifts, but old enough that once that's done, they're off entertaining themselves. My Mum and PIL coming 12pm. DB invited but Mum confirmed last night he won't be coming (read as: he's still drinking). I feel better that DB and Mum knew DB was invited, but grateful that DH and my boundaries are respected.
Happy Christmas. Talk to your Mum about next year. Talk to DP about a plan for next year that respects both his and your (and the children's) needs first, not just yours and your Mums.