Who to Checkin With?
 

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!

As always, you check in with the carrier operating your first flight.

Your itinerary is not clear, but you check in with the carrier that is operating your first flight. This carrier will get you your seat assignments ad boarding passes for all flights to your final destination (assuming al on one ticket). You do the same on reverse.

Welcome to FT!

Assuming that this is a single ticket/PNR and has no stopovers (over 24 hours doesn't count as a connection on an international ticket/itinerary) you check in with UA on the outbound (try to do this with their website just under 24 hours before your departure) and LH for the return (check in for this might open 30 rather than 24 hours in advance of your first segment's scheduled departure time or you might be required to check in at the airport). In both cases, you should get two boarding passes (for both flights in the same direction) at check in.

You always check in with the airline that operates the flight you're about to board. Their staff knows what other flight numbers it was sold as and are used to seeing bookings under those numbers. They will give you as many boarding passes as they can. Those may or may not cover the rest of your journey. If you need more boarding passes for later flights, you get them with whatever airline operates the first flight you need another boarding pass for, at the airport where you will board that flight.

Check in with United on outbound and Lufthansa returning.

They should give you boarding passes for your remaining legs.