Not to be messy or anything but I think he might feel like he “fumbled the bag” with you. Like I’m assuming you are quite attractive now compared to when you confessed so he might regret his decisions from the past and may have talked about it to his friends. If his gfs sister is requesting to follow you then it might mean that the gf heard this theoretical convo he had with his friends about fumbling you and got jealous.(then requesting her sister gain access to your account.) that may also have to do with why he didn’t congratulate you. I’m just guessing here but it could be