Limit On Multi-Select look Up

Limit On Multi-Select look Up

I run a tour company that runs youth sports tours. We are effectively selling 'trips' (products) to families (accounts) which are comprised of parents (contacts) and athletes (custom object). There are some entity relationship dynamics that make setting up the custom object possible but I am running into limitations with multi-Select Look Ups that is limiting my ability to make these relationships. 

I have included our entity relationship diagram for you to see. 

https://drive.google.com/file/d/1hl_YlwhOyvr7zv_R0qfinRhk1-93ep2U/view?usp=sharing

Is there any way to create these connections in the system? Maybe I am missing a function? I am new to Zoho so Custom Functions may be required here too. 

Thanks!