Oddly, my DSD was weighed by her Mum and concious of what she ate whilst we took her on Holiday... she turned out to be bulimic... horribly sad...
I know lots of people who pack ligth when visiting America precisely because clothes are much cheaper there and they intend to bring lots back. Most people I know, even those who are quite conscious of their weight, eat freely and far too much on holiday and gain weight whlst away. Perfectly normal.
Surely their Mother either spoils or neglects them? Is it really possible to do both at once? If your DSS do so much sport they can barely fit anymore in, then they will naturally lose the weight, and as they have probably done none on holiday, that would explain such weight gain.
As someone has stated, 9st is apretty normal weight for a 15 year old boy, and since you have mentioned he is a rower and like to leasure his 'muscles' I would say that he would probably weigh more than most 15 year olds if he is muscular. You or DH MUST weigh these two poor boys often to know how much weight they have put on. Poor children!
I have three DC. I haven't a clue what they weigh. They do eat some junk, but they mainly eat healthily and ar eactive. Why on earth would I weigh them?!