Falling in love? More like falling out of pheromones.
That fake omega is actually an alpha
Both of them secretly had crush on each other but because both become alpha they didn't confess I guess.
Later when the omega brother ran away the alpha brother decided to pretend as an fake omega and live with them
The most make sense theory that i had is both alphas had feelings for each other from the start, but when the seme had his rut, the omega twin is nearby and seme thought him as our uke now and hs with him. Or because Yuuya, our uke, is an Alpha, he used Yuuma, uke's twin brother who is an Omega, as his substitute. Yuuma run away because he realizes that seme actually had feelings for Yuuya and not him. Just my theory tho. Idk if this is true lol
they better give a good reason for the alfa to fall in love with the fake omega, otherwise it'll look like he's only using him as replacement.