"I have the best mother in the whole entire world," Katherine, 21, told PEOPLE. "I could not have asked for a better mom. She supports me in everything that I do. She's always there for me, and we're, like, inseparable."
She added: "I always say we talk way too much. … I actually tell her everything. I should probably stop doing that! Because we're just so close."
Upon arriving at the event – which Katherine hosted at Diane Von Furstenberg to benefit her uncle's charity, Best Buddies International – Maria Shriver greeted Katherine and her friends with warm embraces, telling her daughter: "You look gorgeous."
The proud mom later gushed to a friend: "My little girl. ... It's shocking!"
Brother Patrick 'A Stud'Katherine, who spent most of the summer vacationing with her mom and siblings, is gearing up for her senior year at USC.
"My goal this year as a senior is to act more my age," she tells PEOPLE. "Last year, I was very caught up in doing my book stuff and traveling and stuff. So I think that this year, I'm really making an effort to have a lot of fun with my friends, to go out, be a 21-year-old and to have fun."
Though she describes her aspiring model younger brother, Patrick, 17, as "such a young little stud," Katherine said she has not given serious thought to her own career path.
"My mom always tells me to live in the moment. So I'm just trying to be a senior in college and when it gets there, I'll think about it then."
Dating HurdlesPart of that includes looking for someone to date without her famous family getting in the way – something Katherine said she's struggled with in the past.
"I've definitely had a few guys who you think that you like and you go on a date with them and it turns out that their main priority is asking about body building questions or 'Can I go work out with your dad?' " she says. "I'm like, 'Okay! I see where this date is going.' "
She adds: "I just assume that the right person will come along who accepts me for who I am and it won't be such a problem. But it's definitely a challenge, I will say that."