I guess, you just have to write them down like your own children... or make a note in the way it will be convenient and understandable for you.
I am a teacher in the primary school in Australia, in on one of the lessons we have a task in the class to create a family tree. So it is nice to find this site! I share it with my colleagues from
https://au.edubirdie.com/do-my-assignment and they use it as well!
Wish you best with your tree! :)