That's right. According to the canon list, she has a fifteen-year-old brother who's a Royal, and a fourteen-year-old sister who's a Nobody.
They're both freshmen. i don't know if that's intentional or a mistake, but if it's intentional, I can offer up a few suggestions as to why they're in the same grade despite being a year apart. It doesn't affect Marisol, but I thought it would be helpful.
1. Marisol's brother stayed back
2. Marisol's sister skipped a grade
3. The age cut-off rule. In most American school districts, if you turn five after Sept. 1, you have to wait another year to start school. So if the sister was born in August of 1993 and the brother was born in September (after the 1st) of 1992, they would most likely be in the same grade.
Another thing- Marisol is adopted, and is not in any way biologically related to her siblings. So they don't have to look like her. In fact, they probably shouldn't look like her. Also, Marisol was six when adopted, making her brother and sister respectively three and two at the time. Because they're both girls and in the same clique, Marisol and her sister would be closer than Marisol is to her brother.
That about sums it up, if you need any more info, check out Marisol's bio:
http://z3.invisionfree.com/__wmaa/index.php?showtopic=77