How do I become friends with this random kid next year?

So I am 14, 15 in November and I will be a sopre in high school. I am switching schools also. Well there is this lady (she's in her early 20's) who used to care for my great grandma who ped away a couple years back, but she still comes and visits my grandma every now and then, she's sort of like another daughter to them. Well anyways she got all excited when I told her I was going to that school next year. The thing is, both her and her little brother were adopted, but into different familys. Her brother goes to the school I am going to switch to, but he is like a senior I believe. I get sort of intimidated by older kids to be honest. Well she wants me to keep an eye on him and tell me how he is and everything, but be discreet about it around him. She hasn't seen him since he was 3 and he doesn't really visit her because he doesn't want to be disloyal to his adoptive parents. How do I become friends with this kid next year if I happen to see him?