get hold of a lawyer, and say that you would like to have the government forcefully admit your mother to a rehab center, then tell him/her that you would like to file for custody for your siblings.
you may not gain the custody because your siblings still have their father and grandmother (i presume?). however, if you can prove that you are more than capable of looking after and providing for your siblings, the court may just lean in your favour.
my primary advice, as i said, is get a lawyer and speak to him/her about the situation
then again, if you want your father or grandmother to have custody, that will NOT be difficult considering your mothers current state.
hope this helps