Dividing expenses between siblings - christmas gifts

Hi,

the holidays are coming up, and I'm trying to make an excel-sheet to record and divide all expenses for gifts for me and my sister and brother.
Normally I could program this with ease, but I'm stuck in my own brain and can't seem to think correctly.

As you see in the attached sheet, I have 3 tables: One for input (who paid for what, and who will split the cost), one for total expenses per capita, and the last a calculation form on who owes what.
Here's the kicker: these are separate events, meaning I don't care what my brother and sister owe each other.
For instance, James (me) and Harry are splitting the cost on a gift to Liz. James and Liz are splitting the cost on a gift to Harry. I am only interested to know what I owe Harry, and what I owe to Liz, not the total. Hope this makes sense.

Any ideas on how to make this?