Hmm, was that ever working? Its not covered by unit tests at least (see SearchContextTest->testRelationshipObjectsLinkedInSearch()). Do you have a full example (incl. model definitions) that you can get to work in 2.4.0?
Hm can you provide PHP files with a datamodel (and perhaps some sample data as a SQL dump) so we can verify this? I assume that both "1stDegree" and "2ndDegree" are connected to their respective "parent" as "has one" relations?
BTW, there's another unit test dealing specifically with relationships in search, but it doesn't have the case that you described either - SearchFilterApplyRelationTest