@JohannesLindner @XanderBakker
This solution did work, though now we have run into a different but related issue. We are working with data that is in 3 layers, i.e. there is a related table to the feature class, and then another related table to the first related table, via a different ID field. It looks like this:

The original point table

the first related table, related through stopNo

The second related table, related to the first table through busNo
What we want to understand is, how can run a calculation on the second realted table through the original point layer? For example, if we wanted to know the sum of all the passengers in the buses that went through point A01, how would we do that?
When we use featuresetbyrelationshipname, using the name of the second relationship class, it reruns an empty featureset.
Can you please help me out regarding this?