Ps4 Shares?

As it says above, I have a question about sharing with the Ps4:

If I want to share with my sister the problem is my uncle is already shared with me (he bought a few games that I can also download)

At first I thought I could just do the same thing as my uncle but then realized that it wasn't possible.

If I now activate my Ps4 as the primary for my sister, I would only lose the games my uncle bought or the whole account

You don't lose anything. I also have the primary accounts of 3 friends and I can gamble all of their stuff. A friend also shared mine