I know some people would flat out tell their sister/mother that everything they believe in sucks, but I am not an asshole and I will not do that... I dont know, sometimes being nice can be a real pain.
To be harsh and not knowing anything else about the family, that right there tells me part of the problem might be you.
Rephasing the wording to clarify the meaning bluntly: "Everything my sister and mother believe in sucks, but I'm too much of a nice guy to say it to their faces--just think it."
You don't disagree
with the "some people", you're just nicer than they are and won't come out an say it.
You are respectful of your family's feelings, but not their beliefs. That doesn't sound very "nice".
Now, it could be that the religious side of the family is on the other end of the spectrum in that "you either believe in this and exactly this or you are damned." (I will say that is likely considering the apparent extreme of your position.) If that's the case, you are in a rather tough position with the family.
But if not, then again, part of the problem is your perception. (It sounds as though at least half of the family is not in the extreme religious position if you have a step-father who has a similar point of view. That would mean at least your mother does not fall into that category. Otherwise, he probably wouldn't be your step-father.)
Religious belief is belief
because it can't be proven. There is no more concrete evidence for the non-existence of a "higher being"/"creator" than there is for its existence. There's only one testable way to know if there is an "afterlife"--and the testee doesn't get to come back and report the results. That doesn't mean there isn't, just that it's potentially a one-way door. (You can throw pebbles into a dark hole. Just because you can't see the bottom or the pebbles afterward does not mean the hole is bottomless.)
As an apathetic agnostic, I've had friends on both extremes of the religion debate. Both ends are equally annoying and counter-productive in their fervor.
Not long ago I had a militant atheist friend worried about going back and visiting her religious family...
Her: What if they ask me to do some church thing?
Me: Just tell them, "Thank you, but no thank you."
Her: But the fact that they're even going is so wrong and stupid.
Me: Because what you believe is right
and they are wrong because you're so obviously right.
Her: Yes! Wait, no. That sounds bad...
Now, as to any questions you may have, there probably aren't any questions that man hasn't been asking for thousands of years. You won't find answers (because there aren't any), but you can find plenty of books that discuss the questions (and others you haven't thought of yet) from different sides. There's a whole section of philosophy devoted to religious questions.