Baldur's Gate 3

Obviously spoilers ahead:

I recently got to the lower city and after taking a long rest I was ambushed by some of Astarion’s vampire spawn siblings who want to take him back with them. The dialogue suggests that killing them would close off the option to have Astarion ascend later, but it seems like I can’t avoid fighting with them. I thought maybe using nonlethal attacks would be the way, but upon reading the description it doesn’t work on undead.

What am I supposed to do if I don’t want to kill them? I tried looking up the quest on some wikis/guides, but they don’t seem to give advice on that option. They just mention that if you fail in this encounter Astarion could be kidnapped, which… wouldn’t be ideal considering at the moment I have no spare party members to fill the 4th slot due to… circumstances…but I’d also prefer not to shut off the option for this quest line.