dating a women with 2 kids  need life advice.  please read  women is 31  i am 27?

You're as thick in the head as your writing practices indicate. 
If you're uncomfortable with the fact that she has kids you shouldn't have gotten with her in the first place and you shouldn't continue being with her. You should love her kids too. And you shouldn't "hide" things like this from your own kids. You don't have to act like you're their father if you don't want to but you are a father figure to them regardless of anything because you're the adult male in the house 'best friends' with their mom. Be open with her children, friendly, supportive, encouraging, and help guide them. If you're uncomfortable taking on a full 'Dad' role (you're right, it's not your place to do that but you can if you choose to) you don't have to. Maybe try to think of your relationship with them as the Awesome Uncle. 

When her kids learn that you have kids of your own you're going to be more of a father figure to them. That's a given. Relationships grow unless you prevent them from growing by avoiding people. 

I've been in this situation with a girlfriend I lived with who already had kids that weren't my own. I too was uncomfortable at first but running away wasn't an option that went through my head at all. I just let the relationship grow or remain stagnent if that's what the kid(s) chose to do. I didn't push my way in and didn't attempt being the father figure unless they showed me I was assuming that role according to them. A turning point was when her son asked for me to show up at school to talk to his counselor because he was having problems being bullied. I was there for him as he asked and that kick started our relationship into being a good one. I taught him how to deal with bullies (taught him some self defense techniques but emphasized first and foremost to walk away, only defend if they don't let him walk away), and we bought him a car him and I built together - I taught him mechanics, how to build and work on cars. 
I am a father figure to them and they obviously see me as a father figure but they do not call me "Dad" they call me by my first name.
I'm not their biological dad so I wouldn't want them calling me that if they wanted to or it slipped. If they ever do call me Dad I'm not sure how I would react or feel. 

I stepped up to the plate and continue to give my best swings because the kids asked me to. Their biological dad is a deadbeat. Isn't in their lives and doesn't support them at all. If he would want to join their lives at some point I would welcome him and stay out of his way. I would encourage a relationship with him if the kids asked my advice or thoughts on it. 

That's what I think you should do. Just be there. Let it grow if it's going to. If it's falling apart? Get out of it. 

You need to man up in one of 2 ways:
1. Introduce the kids to each other. You won't know the result until it happens. If you approach it weak and nervously they're all going to be shy and nervous about it and it won't go well. Kids aren't stupid. They know what's going on, even the 2 year old has a good idea about all that's going on with you and their biological Mom and they probably suspect you might be with someone else. A childs mind is wide open to sponge in everything they can learn and figure out even before they're out of the womb. People that think infants and young children are stupid could not be further from wrong. 

You don't know. Maybe all the kids will get along extremely well. They might be excited to have more of their own kind to call brother or sister. 

2. Move out and live on your own like you're considering. Before you do that I think you should consider what you might be walking away from. One thing you need to do if you choose to stay is you need to open up. It's not a healthy relationship if one or both of you are hiding things from each other. You don't have a healthy relationship with your kids if you're hiding this kind of thing from them. Whether you choose to leave or stay you need to fix that with your kids or you're never going to have a good relationship with them. 

3. Be yourself. Stop being nervous and insecure. This woman allowed you to move in to live with her and her kids because you were confident and secure with yourself and you acted like a man. So keep being yourself. Be a man. It's unlikely a woman would let a guy in her home living with her kids if she thought it wouldn't work out between you and her kids. 

Both of you have way too much baggage in my opinion, it'll be harder for either of you to find other partners willing to date you when you'll always have ties to an ex partner and kids that they'll have to pretend they love as much as they'd love their own. Not a situation I'd ever willingly enter, most people wouldn't.

"dont want him to think that i traded him and his brother for two other kids. "

Then act like it.  Of course he'll feel that way!  Stop being so selfish.  You moved in with someone after dating her for 3 months?!  That's idiotic even when a person has no kids.  With kids, there's no excuse for this. 

Also, when you say the issue is you, this is not true.  As if there's not enough else wrong here, she's a crap mom.  This is partly for moving in so quickly, but if she's chosen you "over her kids 90% of the time", this is awful.  Do these kids even have the same dad?  How many other boyfriends has she exposed them to?  Ugh.  Those poor kids.

Harsh words, I know, but you need a wake up call. 

She chooses you over her children?! This needs explaining: she was the one who decided to bring them into the world (jointly) and is responsible (jointly one assumes) for them. She is not responsible for you! Do you not question her priorities? As for your children, I expect that they have friends in similar situations, and if you explain to them that nobody will ever be more important to you than they are (at least while they are children) they should be accepting of your decision. They will want to meet your new family at some point, probably - but they need to know there is no question of any rivalry. 

Good Luck - you come across as being a kind and caring dad.