I think you know if you want to do it. If you don't, don't sweat it. I doubt your bro will hold it against you. If you want to, I don't see what the problem is. If he's 16, he's your bro and he's initiating it, I don't think you'd be commiting a major crime. Everyone has their own feelings about incest but I'm not sure if two brothers fairly close in age j/o together really carries the same weight as, say, forced intergenerational incest, particularly of the hetero nature. I don't think you'd be polluting your gene pool by stroking your dick in the same room with your brother. Sounds more like harmless fun than anything to me.