It wasn't automatically, they have been living with each others for a few months already, way more than enough times to develop feeling.
i thought it was like few days xD
Honestly think it was a few hours at most. He was all already blushy when she got changed.
Right after that point she say that it have been a few months that they have been living together. I am pretty sure between pages 12 and 13 there's a time skip.
Tbh the older brother is my main source of entertainment here. I just cant get on the idea that ML seems automatically in love with with MC yet.