What can i do??

ok, i have two horses[hershey and sinbad] and my grandparents constantly tell me i will not be able to handle them, Sinbad is a great horse. He does not take advantage of me,i trust him 100%, i know he would never take off or buck me off[on purpose] he's a very sweet and loving horse...i've had him for three years now,and they STILL tell me this...now Hershey i recently bought from my uncle in march, we had one incident where he came after me,but it seems he was just testing me,and i handled him ok i guess, and he hasn't done it since and has been good,and he has been gelded and he's sweet and laidback and an affectionate horse,but i've known and been around both these horses since their birth[sinbad is 9,hershey is almost 2]....what can i do to show them i really can handle my horses? and also...i'm not so skinny,and my grandparents and my uncle kind of say things about my weight and my horses[riding] and idk if they realize it,but it hurts when they tell me i have should have a