I feel like he had to be the responsible Brother for 2 years and so had no right to pursue any potential love interests and was good friends with his Sister in law who was the one who took him as a serious good friend,  that now once the eldest Brother is normal and he doesn't have To be the responsible man he immediately looks back to the only friend he has which is the Sister in law. So he starts to have a crush on her . TBH I like their pairing a lot too.    🥰😊