My 13 year old DD does help out a lot...but she does it because she loves to, not just when I ask...so I do hope (after reading your post) that she won't feel like a carer to him.
We struggle with this too. At the end of this pregnancy I've probably relied on the older kids a bit too much and I feel badly about that. It's hard when the kids actually *love* helping out with their little siblings. For instance, dd#4 (who is 9) loves reading bedtime stories to her little sister (who is 5) and that's at a time of day when I am absolutely wrecked, so I've been allowing her to do that... and then I feel awfully guilty that I'm outsourcing my little girl's bedtimes stories to her sister . They often sleep together in the same bed though and are very close, so I guess there's no issue where they are concerned.