Man, there’s too many to name here, incest is easily my favorite problematic fiction trope and it’s how I was dragged into this godforsaken series, but here are some off the top of my head(Note that all combinations of 3 include all ships with just two of the characters):
•Rodrigue/Glenn/Felix
•Rufus/Lambert/Dimitri
•Lonato/Ashe
•Miklan/Sylvain
•Fado/Ephraim/Eirika
•Vigarde/Lyon
•Sigurd/Seliph/Leif
•Quan/Leif
•Eldigan/Ares
•Julius/Arvis
•Greil/Ike
•Ike/Mist
•Lorazieh/Reyson/Rafiel
•Ashnard/Pelleas
•Rajaion/Kurthnaga
•Uther/Hector
•Hector/Lilina
•Elbert/Eliwood
•Eliwood/Roy
•Minerva/Michalis

I’ve never mentioned some of these before and probably even forgot some but that’s because I’m mentally stretched very thin by how many of these games I’ve played lol, feel free to mention any of them to me tho