Does my sentence make sense?
We are interested to see what their relationships will become.
Thanks in advance
"We are interested in seeing how their relationship/s will turn out." (or "work out")
"We are interested to see what will become of their relationships." This means how they will end up; without being concerned with all the details.
If you only mean between two people, use "relationship".
If you have a group of 4 or 5 people, you might say:
"We are interested to see what will become of their various relationships."
"We are interested to see what relationships will develop among them."