In short, Tom and Star are very distant cousins, by virtue of Star's 3rd great-grandmother, Rhina, marrying a 2nd cousin of the Lucitors, John Roachley.

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Let's back up a bit. We've known Star is technically related to Tom since the Book of Spells revealed that Rhina Butterfly was married to one of the Lucitor family's cousins. The discussion came up again a few weeks ago. I myself argued that Star and Tom might be 4th cousins. Someone else said they might be 5th cousins. Either way, that's actually not that close in the grand scheme of things. Nonetheless, I present to you, the Lucitor-Butterfly family tree:

https://preview.redd.it/dvb4eio4b5lb1.png?width=1992&format=png&auto=webp&s=22e05c8b3c576092a16950b80729905e3fd21a25

Rhina Butterfly married John Roachley and had Celena. Their marriage fell apart and Rhina attempted a spell that resulted in John's death, straining relations between the Butterfly and Lucitor families. Nonetheless, their bond would persevere through Celena's bloodline.

So here's my theory (and note, some of this relies on speculation - some of the details are just not there):

1. Demons live a very long time - hundreds, even thousands of years (Relicor stated that he used the Severing Stone "thousands of years ago").
2. Assuming the median age of the mother when each Butterfly heir was born is 25, Rhina and John would have married about 140 years ago (given that Star is 15).
3. Because relations were strained between the Butterfly and Lucitor families as a result of John's death, and Wrathmelior (Tom's mom - the current monarch of the Underworld) seems to be fine with them (apart from Star's booth buddies thing with Marco), the reigning Underworld monarch at the time was someone other than Wrathmelior.
4. Relicor (Tom's great-grandfather) is thousands of years old, suggesting he was NOT the monarch at the time. Therefore, the most likely candidate for monarch is Tom's unnamed grandparent.
5. Therefore, John Roachley is the 2nd cousin of Tom's grandparent.
6. That makes John Roachley the 2nd cousin twice removed (2 generations up) of Tom.

With all that in mind, therefore:

That makes Star and Tom 4th cousins, 3 times removed.

To back this up, I've borrowed this cousin chart (Digging for Ancestors) and modified it for Star and Tom:

https://preview.redd.it/bzxwvh3jg5lb1.png?width=1154&format=png&auto=webp&s=36c1a4b5c7dfdf7fa48e7976e436af2511d90327

Supporting this, Tom and Star's common ancestor is Tom's 3rd great-grandparent and Star's 6th great-grandparent (the same person - through John Roachley). Punching in '3x great-grandparent' and '6x great-grandparent' into this cousin calculator (Genealogy Explained) returns the same result: 4th cousins 3x removed.

And this is also supported by another cousin chart (DNA eXplained):

https://preview.redd.it/oovnyu6qh5lb1.png?width=1397&format=png&auto=webp&s=24514cd937194fbd99d510e51a1974e7def9211e

The top right is Tom's 3rd great-grandparent and Star's 6th great-grandparent. The bottom right is Star.

One more fact, the genetic similarity for 4th cousins 3 times removed is:

~0.0244140625%

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Star and Tom are related, but by such distance that they're practically strangers. Given that real-life royal marriages have been between 1st cousins (12.5% similarity)... Elizabeth II of the United Kingdom and Margrethe II of Denmark were 3rd cousins (0.78125% similarity)... Tom and Star's 'relation' is very, very small.