Unsure Aunt: If mother and daughter are both as "free spirited" as you indicate, living with your practical, organized, dependable self would likely make the daughter rebel even more. You can use your powers of conformity for good, though. Speak to the daughter and offer to get her a tutor. Take her to Planned Parenthood, for the gods' sakes, and get her on Depo or Norplant or something that won't fail if she is flaky. Bribe her for good grades, teenagers respond beautifully to bribery. But by all that's holy, don't tell her you think her mother is an irresponsible drunk who dresses like a teenage tramp. You will never see the kid again.
Once a Cheater: Well, that is an impressive string of poor judgement. I guarantee your fiancee will find out, if he does not know already. So step one: go get yourself tested for every STD under the sun. Then, come absolutely clean to the fiancee. Tell him what happened, tell him who it was, show him the text messages, and take the consequences. He may forgive you, but you will have to work to build up trust between you two again. If he doesn't, at least forgive yourself. Despite the stigma attached to cheating, humans have been making stupid decisions with their genitalia since time immemorial. Next time your boyfriend (this one or another) goes out of town for months, and you are feeling lonely and bored and horny, don't go out with a single sleazy guy friend for some flirting. Go out with a group, or call your boyfriend and have some phone sex, or develop some self control. Learn from this.
Mommy Dearest: Are you a bad parent for liking one child more than the other? I don't care. It is too late to magically rebuild a strong relationship with your daughter. You say she is a polite, pleasant child to other people outside the house. Find her a mentor, a cool aunt or cousin who can connect with her and give her advice. Your personalities may never mesh, but you can treat her kindly and with respect, and when she is an adult your relationship may mend some. Don't expect too much respect from her over the next few years, though; if she has never been nice, 15 is going to be hell.
Philanthropicky: You have given in before, so your friends are not going to lay off anytime soon. If you don't have the strength of will to say "no" flat out, I can't help you much. Make up some convenient lie if you think that would help. Grow a backbone. Donate to a spinal injury rehabilitation center and hope they will someday be able to grow a backbone for you.
Freki