I’m an athiest but I could never understand why other athiest get soo offended when somebody wants to say grace.
I mean whats the big f’n deal? especialy if its a family member and its so obviously important to them.
Print out Matthew 6:6-7* on an index card, and post it prominently whenever your sister will be present in your house. Then when she sets in praying, go all concerned that she is doing it wrong, and won’t get her deserved reward…then shove her into the nearest closet and lock it. 
*King James version:
6:5 And when thou prayest, thou shalt not be as the hypocrites: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have their reward.
6:6 But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father which is in secret; and thy Father which seeth in secret shall reward thee openly.
It sounds like you are looking for Hedonism.
The way to make a lawyer shut up is to sic a judge on her 
Whenever your sister starts talking about God etc., just tell her that you prefer not to talk about this, firmly and politely.
The verses that Beelzebubba and StarvingButStrong provided would look good in needlepoint, I think. Not that she’d get the hint. You might mention that you have enough respect for religion to be upset when you see her using it as a bat to beat people up with.
As a Christian, I like the “your house - your rules, her house - her rules” solution. It is a universal rule of civilized behavior. Make the rules clear when she enters your house. In someone else’s house (your mother’s, whatever) get the house rules clear, and don’t complain when she (or you) follow them.
I’m kind of curious why you think what she’s doing is so offensive. I’m an athiest; my dad is a hardcore bible-banging evangelical fundie. I don’t feel that my beliefs are being denigrated when he says grace, even when he throws prayers in my direction. I don’t even have a problem going to church with him once or twice a year on special occaisons (it’s kind of fun to stand silent while everyone else is singing, gesturing, and falling prostrate before the Lord, in an armchair-anthropologist kinda way).
Your sister’s (and my dad’s) actions in all of these cases were guided by what they truly believed was best for you and your family–she wants your dad and the rest of the family to be with her in heaven, so she’s acting accordingly. It’s actually somewhat sweet, when you think about it.
If you truly don’t believe in god (and I don’t), then what harm is she doing? Is smiling and nodding as she says a prayer that you think will ultimately be inefectual so damaging to you that it’s worth challenging her beliefs and robbing her of some peace of mind?
her dominating things is wrong (your anniversary celebration is your domain, not hers; your dad’s funeral & the family C’mas celebration was the whole family’s, not exclusively hers)- her participating & tossing in her faith is understandable (she was within full rights to get last rites for your dad- if it makes her feel better, why not? how does it hurt anyone else? what is so wrong about saying A prayer at a CHRISTmas celebration? Tho the funeral situation should have included her maybe saying a prayer or reading a Scripture or giving a homage to your Dad, but not her conducting full Funeral mass AS the service. She could have done that as a seperate service.)
I’m with Metacom and Shakes, if other people want to be religious, that is their privilege. Actually, I think it is kind of touching, mostly. It isn’t a contest and I don’t have to bite my lips, just show some basic human respect to minor manifestations.
I don’t much like being evangelized, so I’m careful not to do it to believers.
A grace before a meal is not a lot to put up with.
Realize that the problem isn’t what she is doing (none of which sounds like a major imposition) so much as some sibling thing you have going–you seem to feel she is trying to one-up you or something. If she is your zany leaps-in-with-both-feet sister, then she is, and love her because she is your sister. She is never going to stop being zany.
What you can work on is having a stronger sense of self, and seeing that what she believes is not impinging on who you are unless you, yourself, want it to.
Would it be worth it to go to her superior(s) in the church (bishop or somebody—I’m not sure how the Episcopal church’s hierarchy works) and ask that they gently admonish her not to be so pushy with her religion?
I’m not an aetheist but I’m not into organized religion either. I just have my own beliefs and basically what other people believe is okay with me. We had a Christian funeral for my dad and it didn’t make any difference to me because I don’t think human rituals have any spiritual relevance. I understand why you are annoyed with your sister because she wants to convert you but I really don’t see any real harm in anything she has done except be as annoying as hell; my sister went to a Baptist church when she was 15, ended up marrying a future preacher, and now, at 50, is still a bible-thumper. Many times over the years they have ganged up on me and tried to save me; but the only time I got furious was when they took my young children in the bedroom and told them they were going to hell and scared them to death. When I was a Philosophy major in college my brother-in-law used to point out passages in the bible about philosphers being of the devil. Mostly it amuses me. The part that baffles me is how such intelligent people could totally ignore quantum physics and put so much faith in the simplistic ideas of over 2,000 years ago. Belief in Heaven and Hell and Sin seems so childish. I let them bow their heads at Thanksgiving and Christmas at the table and let them do their thing. Over the years they have mostly given up, but of course they still think I’m going to Hell. By the way, when my sister used to plop the bible down on the table and start asking me if I was afraid of going to hell, and then use the bible as her reference, I would tell her that I was sorry but since I wasn’t a Christian I couldn’t possibly accept agruments based on a Christian bible, but that if she had any other source that I could accept, I would be happy to entertain her ideas.