Claudia wasn’t born a Phantomhive but married one (like Rachel) which means Cedric was her lover (since his name isn’t Phantomhive). In this case, I should show that Claudia’s parents’ names don’t look like “Phantomhive” but since they’re really illegibly written it’s not a proof.
In any case, Cedric is definitely Vincent’s father and he had 66.5% of probability to be her lover and 33% of probability to be her husband, while Claudia, as a woman in this era, couldn’t dream of staying single since she had to bear her husband’s heir or her own, according to whether or not she was born a Phantomhive.
That’s when it gets complicated: If Cedric is Undertaker (because he shares similarities with Vincent, because he had a link with Claudia, because only one glance was needed for Ludgar to realize Ciel’s lineage and Cedric is the only one fishy on that tree, with his info hidden) do you really see the Undertaker marrying and acting as a Lord? Seriously, I don’t.
It just wouldn’t do, in my opinion (even if he did make a very convincing Principal in Weston).
Which is why I hesitated on the marriage if Cedric is the Undertaker. That doesn’t suit him.
We could also consider that Cedric isn’t the Undertaker of course (even if Cedric is the reason behind Ciel’s particular lineage, that’s obvious), but I really find too many similarities between him and Vincent, him and Frances, and he’s got that link to Claudia, Vincent and Ciel…
Sorry that last point wasn’t really helping :/ but I hope the rest is understandable enough! :)