Laura,
You will find the strength, trust me.. You don't have to say anything, just being there for your sister is enough believe me. There were plenty of times I felt like a jerk, not knowing what to say or do, if it was the right time to give her a hug or not. Its o.k. if you feel her friends are more supportive with words than you because remember this is your sister we are talking about, someone you spent a lifetime with, so its easier for them with words because sometimes we don't want to face the reality and being that your sister is a christian like you say as mine was, I expected her to be GODS one true MIRACLE and so did she, and that might make you more uncomfortable. So, my advice to you is find your strength in your sisters faith, thats what is helping me get through my sisters loss, because its is the hardest thing I am facing right now but by the closeness she bought me to God brings me alot of comfort these last few weeks since her passing.
You and your sister are in my prayers,
Andrea C.