Step parenting

You also have to be very careful not to judge the living situation at the mother's.  You want your living situation to be accepting, which includes accepting what she has to go through at her mom's.  If you can do this without creating any animosity between the mother's situation and yours, the daughter will start to make good choices on her own.  As Windy city put it, you can be a fantastic role model and hopefully, she'll gravitate towards that.  

Don't feel you have to 'whip that kid into shape'.  that will also create animosity.  Acceptance, love and boundaries will get you far.

Pink is correct about not feeling you have to whip her into shape and judging the mother's living situation. There are 2 sides to everything and if the little girl feels you are judging her Mom she will really put up a wall. Never make the mistake of criticizing her Mom or talking bad about her in front of the child THIS IS A BIG MISTAKE for you or her father. It shows a complete lack of class on your part and can be damaging to this little girls self esteem. Maybe she doesn't know her Mom is shite.
Jess, I know step parenting is a scary thing but it can also be very rewarding too. You get to help raise your BF's child with him which will bring your relationship to a whole new level not to mention you get to be this little girl's really good friend. Someone to do girlie things with like painting toenails and trying out new hairstyles and such. Mom's sometimes don't have the time to do the little things because they're too busy with normal day to day life like getting them off to school and making dinner and homework. You won't be parenting full time either which for you seems is the ideal thing as you can kind of slide into it gently.
There is nothing like the look in the eyes of a loving trusting child. It is magical and pulls you in like nothing else. It wraps around your heart and makes it shine. Things have a way of working out. Hang in there really.

Thank you windycity and everyone else.  I don't think I'll ever be 100% comfortable with being a step parent, but I'm going to go with it and hope for the best.  *fingers crossed*

Good for you Jess. I wish you and your new family health, happiness and love.

C