You could use "... 말고 ...":
이거 어머니에게 가져다 드릴래? (아니) (그러니까) (내 말은) 어머니 말고 네 동생.
As mentioned in the comments above, several phrases such as 아니/그러니까/내 말은 can also used to mean "I mean."
However, they all have other uses, so they can be ambiguous.
So, I would recommend "아니 그러니까" or "아니 내 말은", although they feel a bit verbose. (Even they can be used for something other than "I mean", but I feel they're more likely to be understood as "I mean".) To recap:
이거 어머니에게 가져다 드릴래? 아니 (그러니까/내 말은) 어머니 말고 네 동생.
You can even say all three at once ("아니 그러니까 내 말은") though that may sound too talkative (sort of like "Err, no, I mean...").
If you are aiming for brevity, I think this could work:
이거 어머니에게 가져다 드릴래? 아, 어머니 말고 네 동생.
("아" is commonly used to mean something like "Oh I just realized...")