I have 9 and 5yo boys. I cannot imagine expecting the 9yo to physically bathe his brother. They've bathed together a few times the past year. But, my 9yo does not touch his brother in that way. With both of them, I have showered with them up to age five, at which point I began teaching them to wash themselves. My current 5yo can now do a fairly good job on his own but I still help him sometimes or shower with him if we are in a hurry. As others said, they only bathe about twice a week, though the 9yo is starting to need more. But at 7, 5, and 3, they should be fine with washing hands, feet, and faces most days.
I also don't have the older do diaper changes or potty help. Both because of the intimacy of those acts and because 9yos (and younger) aren't the most meticulous.