How can you be someone's aunt AND grandma?

I found out about an upcoming episode of UncleGrandpa and it doesn't seem to make sense to me. Even Mr. Gus doesn't know about AuntGrandma. How can you be a aunt AND grandma at once?
https://www.youtube.com/watch?v=hGTQxmC_XyY

N of C, b. '532014-09-30T14:32:45Z

Favorite Answer

Well, if your sister has a child, that would make you an aunt, and if you then marry your father, voila ! AuntGrandma.... Of course, to make it a little less creepy, you could marry your brother-in-law's father with the same result.

bennett2014-09-30T09:45:16Z

Aunt grandma and uncle grandpa are essentially Gods as far as I'm concerned.

?2014-09-30T09:53:11Z

Your grandchild marries someone who is your spouse's niece or nephew.

?2016-09-18T08:42:05Z

It is an interesting question