FINAL UPDATE: He is not improving at all. My mom still told me that he is upset because of her, not me (which I still have a doubt about it). He doesn't like to share out his feeling and always kept his thought to himself (this is the problem that me, my sister, and my parents are frustrated about because we just don't have a clue). I want to try to help, but it seems like he doesn't even want it. Over these past weeks, I began to take care of myself and slowly let go of that situation instead of holding that toxic thoughts and forcing myself trying too hard to cheer him up. During the past week, we are all becoming silent and not talking to each other (and he still hides in the store when going to many grocery stores). Even if I tried to ask him, he just won't still respond. This led me to believe that we are not close anymore, but this helps me get closer to my sister more (she is the only person that I can talk to). However, it doesn't mean that I won't care for him anymore. In just three days, I will be leaving the household and I can set my worries aside, for now, to focus on studies until my mom can finally coming home (October 3rd). I expect this will continue on when I leave for college and we can't change that fact. I understand that he is a teenager and he has many ups and downs and he needs parental support. Only my mom can talk it out to him (I still have doubts that this won't work on him because whenever my mom talked about me and my sister, and her leaving to Vietnam, he became sulky and sensitive and trying to avoid this topic and switch to a new topic like no problem). I just hope that when I coming back home after school (probably the next three weeks or so), he will finally turn back to his normal self. I don't have my hopes high, but there is always light through the tunnel.