Lusikkamopo

Ruthven as his father

First of all, they share some ressemblances ; red hair, nose shape, general face complexion.

Secondly, we know Ruthven used to have a belief similar to Noé : he loved both humans and vampires, and used to think of them as equals.

Chapter 31

Therefore, it wouldn't be a stretch to think he might have fallen in love with a human in the past, or at very least, had a relationship (sexual or more) with a human.

Lastly, his reaction to Dante during chapter 65 could be interpreted as annoyance- Marquis Machina calling him to bring up his past in the form of Dante, his child.

Ogier as his half-brother

We know they cannot be brothers, as Ogier is a human. However, they could be half-brothers, or perhaps related in some other way (cousins, maybe).

Firstly, we know Dante's mother comes from a (more or less) wealthy background ; otherwise, Dante wouldn't have been educated (it is at least very unlikely), like the other dhams. But he knows how to read, write and calculate. He also seems to understand the general way society and economics work, and how to take advantage of it, even starting with nothing. All of that from a young age, as he is a child when he is abandoned.

My baby...

We also know Dante was likely abandoned in the human world (and not Altus), which reinforces that his mother was human (and therefore, his father was a vampire).

Ogier also comes from a wealthy background, otherwise, Gano wouldn't interact with him (as he openly despises Roland for his modest background) and chapter 67 confirms it again. Therefore, Dante's mother could very well be Ogier's mother, too.

Additionally, they look alike. Even Ogier's younger brother (the child to the left) looks the same as younger Dante did. He's even wearing similar clothes as Dante had.

Side note : While characters looking alike doesn't always mean something, I will point out that MochiJun is particularly talented in avoiding the 'same face syndrome'. All of her characters are unique and distinct. Therefore, characters resembling each other might not be a coincidence, imo.

Thank you for reading! Feel free to add anything to complete this theory!