The good stuff
Tyler is a loving, selfless older brother, putting his little sister's needs above all else. He loves her so much, he's willing to defend her, to try and force their father to acknowledge and support her. However, he also starts off his relationship with Ally on false pretenses and doesn't tell her truth for a long time. Ally is forgiving, even when the men in her life hurt her deeply.
What to watch out for
Although the overall message is about love and forgiveness, the majority of the movie is spent chronicling Tyler's self-destructive behavior and the emotionally distant or borderline abusive relationships between grieving fathers and children.
The movie starts off with a shocking murder -- a mother is shot by young muggers right in front of her 11-year-old daughter -- and violence continues throughout the movie. There are several fist fights that leave characters bloody -- in one instance a man nearly strangles another to death. A father strikes his daughter. A woman slaps a man across the face. A man pushes a schoolgirl's desk and throws a fire extinguisher through a school window, frightening all the students and the teacher in a class. A young man's suicide isn't shown, but it's frequently alluded to and explicity described. A young girl's hair is horrifyingly cut off by spiteful girls at a slumber party. An unforeseen and seriously traumatic event leaves all of the characters grieving and broken.
There are several love scenes that show the main couple kissing passionately, breathing heavily, moaning, and obviously making love. A couple of the scenes show a lot of skin (bare backs, legs, shoulder, and arms tangling in sheets) or Ally in a bra, but there's no actual nudity, and the emphasis is on the couple's faces. In addition to the sex scenes, Tyler's roommate discusses sex and relationships on a regular basis. He makes jokes about Tyler having a way with ladies and his own accomplishment of having "bagged" a girl from every continent. Ally's father accuses wealthy Tyler of slumming it with middle-class Ally ("having a little vacation in coach before heading back to first class").
For a PG-13 film, there is a surprising amount of strong language: more than one "f--k," plus frequent "a-shole," "pr--k," "s--t," and "Godd--n," and the occasional "p--y," "jerk," "d--n," "bas-ard," and "bi-ch."
Not an issue.
Drinking, drugs, & smoking:
Tyler smokes cigarettes in nearly every scene, and it's an ongoing reference in the movie (everyone comments on it). College students drink at a bar, at parties, restaurants, and at home. One night Ally drinks so much she gets sick in front of Tyler and basically passes out on his bed. It's implied that Ally's father could be an alcoholic.