You also need to keep in mind that your brother is at the beginning of his sexual peak, so that in and of itself is a strong force to keep him with her. Enter our writing contest and win lifetime royalty payments. I meant Angelfacegirl. She asked if age mattered. Age does not matter when there is love, or at least, not in the present.
Love has no boundaries.
At the same time, I beleive that one thing most people expect of love is for it to last forever, in a shakespearean, undying love kind of way. Love can be harder to keep alive in certain situations, for example, one person going through all kinds of life changes while another is settled down. This can apply both ways. People always think of this as a young person changing while an older person is settled. Used under License.