Whoever in "Tech Support told me it's supposed to default to the closest blood relationship" is wrong.
It does *not* default to blood relationships. It defaults to 'closest connection' no matter how many marriages it goes through, interpreting a spousal relationship as equal to a sibling or parental relationship.
It is a frustratingly poor widget.
The relationship calculator in The Next Generation (TNG) tree software, on the other hand, shows actual lineages with bloodline preference. Try to find a tree hosting site running this software (such as MyRootsPlace) that has the people you are interested in. It **is** possible to design a useful relationship calculator.
Example: in one of my lines 2nd cousins married and are my ancestors. TNG software shows each one's lineage to their common ancestor. Ancestry.com's presentation only says they are married. Duh.
This problem has existed since the thing was installed. They are well aware of it. They just have not fixed it.