How am I suppose to make my sister and mum feel better after our fathers death?

you can try talking to your priest or rabbi, or whoever you are connected to;; things might change a little after the funeral;; evidentally, your mom is not in an area that she can explain things to your little sister;; you need help to do all this; again, your priest, a family member like grandma, grandpa, uncle, aunt or really good friend of your dad's;; if not, sit down & talk to your mom & calmly tell her the family needs help to get thru this, even her;; it may help you to try & remeber the good things about your dad & not how you saw him the last time... good luck