Older sibling sitters

I'm a little hormonal right now but this really gets under my skin. My older son will be almost 11 years older then his lil brother. When people find this out the most common response is "oh you'll have a built in sitter".....worse is when some point out how there 12-17yr old are constantly baby sitting younger siblings.

This is i guess a pet peeve my son did NOT choose to have a child or younger brother NO he is a child himself. On occasion when his brother is 5+ I'd be fine with him watching him on rare occasion. But he is not a built in sitter i can't see pushing this responsibility on him he isn't responsible for his parents decisions. Maybe i am over reacting but it just seems wrong.