(I have yet to finish vanilla Skyrim).
LOL, I know the feeling--too much time spent modding, and waiting for other people's mods.
Fortunately, I don't actually need the follower to be in the faction, I just used that faction to find the followers to get them to board. I can put any conditions I need to fill the alias. So if you saw some other condition, that's all I need.
Hmm, if you're looking for simple conditions you can use to fill an alias, how about being a member of the PlayerFollowerFaction, as you have, OR being a player teammate (via the GetPlayerTeammate == 1 condition)? That should encompass Serana. (With additional checks that you probably already have to ensure the race is a playable race, so pets don't get seated, etc.)
The thing is, if you don't have Dawnguard, you won't be able to directly set conditions based on it. At best you might be able to use GetFormFromFile to fill an alias you create, and then test based on that alias.