When to come out to parents?

Hi guys, I just been thinking about when would be the right time to tell my parents that I'm dating my best friend (a girl). Me and my gf meet about 4 years ago and we were just friends then. (at this point both of us considered ourselfs straight) But we became very close very quickly. About 2 years ago I started to realized that our friendship was becoming more than just a friendship, and was afraid that I might have feelings for her. This went on for a year and slowly progressed and we became closer and closer, anyway the point is a year ago we started officially dating. (Our 1 year anniversary was last weekend! :eusa_danc) I couldn't be happier and we have already been talking about long term stuff. Right now the only other people that know is our best friend and her boyfriend. They are both very supportive and have many other gay friends. We've even gone on a few double dates. Currently I'm off at a university and have been for year now and my gf is back home at a community college(3 hours away). We skype/call/txt/everything possible, and I come home everyother weekend to see her and my family.

My question is, when should I tell my parents and/or her parents?? Or should I tell my sister first (I'm very close to my sister, and not very close to my parents). I have a feeling my parents would be supportive, and I have a feeling her parents would completely disown her. My family already accepts my gf as a member of the family but they only know that she is my best friend. She is always at my house. Basically lived there over the summer when i was home. We share my bed when I'm home, which might have to change if I tell my parents. (kinda afraid they might wonder if we've had sex if tell them we've been dating, which we have, and they wouldn't like that at all)

We've been talking about that in a year, when my gf finishes her community college degree, that she'd come live with me, where ever I may be since my career will make me move the first few years till i get settled somewhere. I feel like my parents should know by then that we are dating cuz otherwise why would my best friend be living with me and following me around like a puppy. But I'm terrified to tell them. Even though my parents are very loving caring people and already love my best friend like she was their own daughter, I'm still scared that they wont accept it, and they will try to change me. My parents are Catholics but not very strict ones. I mean my mom likes watching the Ellen show and I'm pretty sure she is ok with the fact she is gay, but im not sure. Also my mom has a tendency to know EVERYTHING and just not tell you she knows. My dad is usually blind to things and probably wouldn't ever figure it out on his own. I've extremely close to my dad. For him to be unexcepting would destroy me. I picture him at my wedding and i want him to do all the normal dad stuff.

I don't know when the right time to tell them would be, or even how i could do it. Telling my sister would probably be easier than my parents because i never talk to my parents about personal stuff or anything like that. The reason i havent told my sister is because 1 I just dont know if im ready to tell more people cuz im happy the way things are now, 2 because she cant keep secrets. She would NEVER tell someone something on purpose, but if mom or dad were to ask her she wouldnt be able to lie to them with a straight face. For example she has 4 kids now and each time she was pregnant she tried to keep it secret till her and her husband could tell everyone, but my parents always could tell.

So do i tell my parents? Or maybe tell my sister then my parents so my sister can be there to help me tell mom and dad? Or just not tell them yet? When would i tell them? and HOW?

Sorry for the long post and so many questions. That's my whole story and everything I've been thinking bout for a long time. Any advise?