Hi Dibdob
Christmas certainly is an emotionally charged time and I think that's why its so triggering for people with BPD-if a negative emotion is triggered they just find it so hard to regulate it.
I think it helps if you can keep things as cool and low key as possible.
Also make little bits of time for yourself if you can-its very easy to get frantic and wound up ourselves at this time of year-which not only isn't nice for us but doesn't help the BPD person.
I think its best to assume that something is likely to kick off and be prepared for it rather than hope to avoid all aggro.
Validate the valid if you can eg if the BPD person is distressed because they are missing someone.
Too much alcohol fuels the fire. (Having said that if it really gets too much slope off and knock back the wine in private

)
Good luck (we will all be needing it)-hope everyone has a relatively peaceful Christmas-mine can't be much worse than last year.
We have each other to turn to on here too.