But a baby is too big to fit in a can...
I think it's important for older siblings to understand that the reason that adults do that is because the younger sibling previously couldn't do those things. It's not that people are saying that there's anything inherently special about being able to sit up, but that there's something special about this person sitting up. And while it's important to pay as much attention to both siblings, just patronizing the older child and feigning interest in their ability to do things like sit up isn't really a good long term solution.
Message
Older children can do the same things babies can do.