Don't feel guilty! You make the best choices you can in your circumstances, and there's no point in regretting the things that don't get chosen. Besides, a sister's graduation is surely a much more inspiring event than the NUMATS ceremony! Does he get to go to it, or just to hear about it? You don't say what his sister studied, but maybe, if he'll be there, she could show him briefly round a department that does something he's interested in (maths?) or ask another student to do so if there's little overlap between their interests?
