You can claim your nephew because he is a "qualifying child" if you have been paying the support for more than 6 months this year (I wasn't sure if you were working and supporting them while your mother was still alive). If you are single you can claim head of household which offers more benefits that filing single since you have a dependent. If your mother was also completely disabled, then you could claim her as well as a "qualifying relative".
Your sister would have to be disabled to claim her, but you should get a break with the nephew and your filing status.





