Under what accounting head should this account be created?

[Mike or Penny Novack]

for example me and another family member of mine pool in money to buy a
property as an investment. and when the price goes up and we sell it. then
we will share the profit.

That it's a relative isn't relevant:

a) The investment itself should have a set of books (for the 
partnership/joint venture. Look up "accounting for partnerships".

b) On your books, your equity stake in this investment is an asset.

c) This isn't a gnucash question as you would have EXACTLY the question 
no matter what you were using to keep your books.

Michael D Novack