Mental disorder. With please guide.

Dear sir/madam My middle younger brother (22) is very slow. Even now he can not calculate simple sums of math. We accepted as he is and never forced anything on him and thought by time he will be ok. 3 years ago my mother passed away which left father, youngest sister (18) grandmother and 3 of us brothers.Me and youngest brother are mostly away from home bcz of study and job .Sister is taking care of family since then. The middle brother and youngest sister had a lot of altercation since they study in same class  we though it is normal. In one of quarrels sister broke his phone and he hit her . Father some how took sisters side. Since that day he considers sister his greatest enemy. I gave him  another phone but still he consider her and father his enemy. From few days he is talking like I will kill her and afterwords will sucide or will go somewhere away. Please tell is there any kind of treatment for him. What type of specialist I should consult with Please guide.