How do youspar with someone who is a complete coward and flinches every time you ex a bicep(my brother)

I think that's the easy part
The hard part is how do you spar with someone if you flinch every time.

I have the same problem with my younger brother. First of all is he much smaller than you? My brother is and you have to look at it from their point of view. Also does he like boxing? My brother doesn't really like boxing and he doesn't want to hit me. You can't make someone like anything. Also remember sparring is for learning purposes and not to hurt someone. It is not a competition so take it easy and try to take something away with you from each sparring session.

Why would you even consider sparing someone like that?

You spar to get better, spar the best fighter you can, not the crappiest.

Perhaps due to lack of other opponents? If I could find even a crappy fighter to fight I'd be happy because I can't find anyone to fight except for during the training.