"I'm my own grandpa" – is there a word for this?
I'd suggest kin to self.
Kin refers to one's related people, and is often used to describe family relatonships in academic contexts.
I don't know of an existing single word that conveys the same meaning. Maybe self-kin? :-)
I never heard, read, or came across anything, any word, any term for this state. I also searched and found nothing. For example, see WolframAlaph (the Computational Search Engine) and you often see or search for the genealogy description, not a single term.
Also you can take a look at these pages:
- Genealogy Relationships
- What are relations?
consanguine (or consanguineous) is the closest I can come up. It’s an adjective meaning “of the same blood line”, “descending from the same ancestor”. Derivatives include consanguineal (adj.) and consanguinity (noun). A marriage with one close to yourself in your blood line is called consangineous marriage.
All that is about blood line. I don’t think (and couldn't find) an similar term for the formal relationship of marriage, as the one you’re looking for, except for the first degree (called “in-law”). My husband’s brothers are my brothers-in-law, but that doesn’t extend to the rest of the “network” formed by marital bonds.