A brother’s responsibility towards his siblings at home
Question

                My father works overseas, and I am the oldest son in the family. I live with my mother and siblings. Am I regarded as responsible for my siblings, in the sense that I should enjoin them to do what is good and forbid them to do what is evil, and do they have to obey me in that, or is this my mother’s responsibility? If my mother falls short in that and lets my siblings do some evil actions, what should I do?.
Praise be to Allah.Enjoining what is good and forbidding what is evil is obligatory upon you at all times, whether your father is there with you or he is absent from the home. It is also everybody’s responsibility, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever among you sees an evil action, let him change it with his hand [by taking action]; if he cannot, then with his tongue [by speaking out]; and if he cannot, then with his heart [by hating it and feeling that it is wrong] – and that is the weakest of faith.” Narrated by Muslim, 78. 
Based on this, it is obligatory for all Muslims in general, and on fathers and mothers with their children in particular. If the father or mother falls short in that regard, then it is obligatory on the son, whether he is young or old, to do that to the best of his ability, with good manners and wisdom. Allaah says (interpretation of the meaning): 
“So keep your duty to Allaah and fear Him as much as you can”
[al-Taghaabun 64:16] 
And he also says (interpretation of the meaning): 
“Allaah burdens not a person beyond his scope”
[al-Baqarah 2:286]
You should pay attention to the best way of achieving the shar’i interests, and if the evil you are trying to ward off is greater than any evil that may result from your speaking up, then you should go ahead. You have to show them that you care about them and want what is best for them, so that this will make your advice more acceptable and they will do what you tell them, without doing that in a harsh or suppressive manner. 
We ask Allaah to help you and to set your affairs straight.