Help needed

we are 5 sibllings altogether.2 brothers and 3 sisters.my father died 8 years ago due to heart failure. my 2 elder sisters who are married and one brother are behaving in an abnormal manner.they talk to themselves, laugh loudly , become violent suddenly without any reason, abuses alot and uses bad language alot even when they are alone or in public. they dont take care of themselves. just walk to and fro all the time.they also have bad eating habbits. they eat alot in an abnormal way but still they are always hungry. they have negative opinion about every one and thinks that all are after them for money.they always try to copy others. they sometimes talk sensibly, can drive as well . me and my other brother (we both are younger than them and are just 20 and 17) are very much wooried about this situation. we have talk to many psychiatics.some diagnose depression,some says its schizophernia or bipolar disorder. after medication they keep normal for 3-4 months.while my brother is not willing to take medicine.infact he has tried to injured my younger brother and my mother several times. we are living in an environmennt of fear. what is ur opinion about them. are they really psychiatic patients or they are doing all this just to annoy us. plz tell me what should i do now..our relatives are also not very supportive in this regard and we also have no other place to go

if he gets violent then a psychiatrist is necc,counseilling therapy required urgently,try to make him see the positive side of life,he may be depressed try to be kind he will open up and solve his problem