This is so sad... Jaemin's mother thought that she would be relieving pressure from him if she told him that all she wants is for him to be happy all the time but instead, the way that she said it just pressured him more to fake being happy all the time...
The mom is doing what she did to the older brother to Jaemin. He hides behind a smile and little by little takes it all until he cracks. He would have probably gone down the same route if he didn't have his friends.
I don't think the mother ever realized that she's doing exactly the same to his brother who couldn't open up and rely on someone else, neither his friends nor his family/mother. I feel so sorry for Jaemin..