The age difference won't be so big when you are older, but I can understand your brother being concerned now ~ because 17 is still young. What is the age of majority where you live?
Your brother knows things about this boy that you don't ~ possibly about past behaviour with other girls for example.
Twenty-six-year-old men sometimes want something different from a relationship than do 17-year-old girls.
I'd be cautious if I were your Mum. Your brother may see a different side to him from the one you & your Mum see. It might be fine when you are older and wiser, but your brother may just think that you are not mature or experienced enough to deal with this man just yet.
