So, goofy question. I booked airfare to Copenhagen via United. In the end, the only legs of the trip that are actually United are to/from my home airport to the nearest International airport. The other legs are two on Brussels Air going to, and the first two legs returning are Lufthansa.

So, weird question. Typically, who would I checkin with? I mean, its obviously United going out as thats who services my local airport but would I need to checkin for all airlines involved or just United since they are the holders of the reservation? I have already contacted Lufthansa and Brussels Air to get seat reservations lined up as I am travelling with my 72yo mother and wanted to make sure we didnt end up on opposite ends of the plane, I just didnt think to ask about the checkin issue until today.

Thanks to anyone who chimes in!