I recommend checking the walkthough on the first page because my memory is a bit rusty on this one. Still, let me try and remember, because this one is more complicated than the rest.
First, you have to visit your dad in prison, then find the lawyer chick and advance with her until she tells you about your father's second family. After that, I think you might need to get the package for your mom from the mall, and meet the youngest step-sister there (not really sure if this is necessary). Once you know about the second family, you must visit them and even though the stepmom will be angry, the other two sisters will meet you as well. Then, you have to wait for the weekend and find them at the mall, where they're having lunch and talk to them. The stepmom will still be pissed at you but the stepsisters will invite you to dinner. Once you go to dinner at their home, you can start building the relationship with them as well.