Hi, i'm new here so i'm not really sure how to start off..

I live with my boyfriend and 2 of his 4 children. The two children that live with us are 9 and 11. His other two kids live with their mom and they are 13 and 15. I've had previous problems with his oldest daughter that is 15 which caused her to move out of our house and back to her moms. I do not have a problem with any of the other kids except his 9 year old daughter. I hate to say anything mean because i know she's just a little girl, and maybe she doesn't get that the way she acts gets in between her father and I and our relationship. But I need someone/ something to vent to because i'm going CRAZY!!!!!! I've never had any feelings like this towards anyone in my life and it makes me feel like i'm such a horrible person but here it goes.... I really can't stand her. First of all, she is so rude and has no manners at all. She doesn't flush the toilet or wipe her butt properly, hardly ever washes her hands, burps, farts [at the dinner table!!] and says really rude things to everyone and makes funny of anyone or anything that she doesn't like. She has the most annoying laugh and sometimes I really just can't stand to be around her or even her voice!! She has long beautiful red hair but the whole "red hair" thing makes me think that everything anyone says about redheads being crazy is true!! She thinks she is the cutest thing to ever walk the planet and she's a total brat most of the time. People praise her for her red hair whenever we are out in public and all i can think is that people are crazy because they don't know the real her! Don't get me wrong, I love most things about her. She can be so caring and sweet when she wants to be. My boyfriends other 3 kids don't like her very much either because they feel that he favors her, and HE DOES! He says he doesn't, but you can totally tell just by the way he treats her. He gives her whatever she wants, never corrects her for anything she does wrong, and when her and her brother fight she is the only one that doesn't get in trouble. She picks on her older brother [11 years old] and he really dislikes her! She follows him around, copies everything he does [which i know is normal for any younger sibling to do with their older siblings], she hits and kicks him all the time for no reason, makes fun of him because he has a problem still with peeing the bed. When I wake up in the morning and I come have my coffee in the living room, then I here her bedroom door open after she wakes up and I just want to go back to sleep. Recently, for summer vacation, my boyfriends sister took my boyfriends kids to spend the weekend at her house [only the 9 and 11 yr old]. She said that my boyfriends daughter was being a brat the whole time and she told me that she thought she has an "evil" side to her. I hate to say it, but it's so true!!! So the next weekend after that, his sister took only his 11 yr old son on a camping trip and left the daughter here with me. Her own aunt didn't even want her to go on the trip with them!!! Whenever me and her argue, or she doesn't listen to me, i get so upset I have to hold all my anger in and just let it go. But sometimes when i do lose my cool and i end up yelling at her and sending her to her room, my boyfriend comes out and tells me im picking on her and that im too mean to her! He says this all right in front of her, so she thinks that he is taking her side over mine and she LOVES IT!! after she gets her way, she will walk around the house with this evil little smirk on her face just laughing and grinning. UGH!!! I GET SO IRRITATED WITH HER AND I DON'T KNOW WHAT TO DO!! does anyone have any advice? or need to know more to help me out??? how can i change my feelings for her so i can actually have a good relationship with my boyfriend and someday get married. Because right now, with her acting like she is, I don't want to get married any time soon. Advise, PLEASE?!