As long as you're not giving her a big tongue kiss - I don't see anything wrong with a peck on the cheek and an "I love you" from a brother or sister or parent.
I'm 40 and my brother is 38, and we can give each other a peck on the cheek, a hug even, and an occasional "I love you". Same for my sister, and my mum.
The trouble with the English language is we have the same word "Love" for quite a few different varieties of affection and caring.
I love you like a brother/sister
I love you like my dad
I love you like a lover/partner/spouse
I love you cos you helped me solve a particularily frustrating problem
etc etc etc.
I guess if you don't feel comfortable with the whole thing - you probably would have to explain to your sister that you still think she's the best, but now that you're older, showing affection just feels too weird. Maybe by the time you're 30 you will feel ok about it. Maybe you could think of a way of showing your sister affection that doesn't feel too weird like doing something nice for her or sending her an occasional gift or card that she'd like with your appreciation all written out. Hmm, personally - I find the quick peck on the cheek less awkward.