Ve.nus

It's because he's a really young kid, it's not literal.
It's like an endearment + it would look more appropriate. It's hard to explain that's how it works for my end too.

My aunt is only 1 year older than me so I don't call her aunt, instead just act like our age and our relationship is more of how cousins would than an aunt and niece.

It could just be a translation thing or cultural. Like he my be calling himself grandpa like hyung is used for older guys but is not necessarily related. Or in a cultural aspect I know my culture is different from Korean culture but I don't call my uncle uncle because he is my age. Also Euiyoung is treated more like their child then brother so I can see how they can be called his grandparents

In Korean, terms like, brother, sister, uncle, aunt, grandma, and grandpa can all be used for both blood relations and non related persons based on age. Any kid Euiyoung's age would generally be told to refer to elderly men as grandpa. So it just has nothing to do with how they are related here. The man is old enough to be a grand parent and so he automatically falls into the category.

it is cultural probably you dont need to have blood relate to call someone aunt uncle grandpa etc.

Yeah, Jerry is also mad uncomfortable with his brothers being candidates, though I'm pretty hopeful that author won't do genuine incest, so far it's only been played out as an obstacle for Jerry to avoid like the plague.

I like to think that the brothers still love him in a familial way as long as an event isn't triggered.

I kinda like it in a meta way. Like the author acknowledging how incesty yaoi is, and otome games...and shoujo isekai...so much incest. But it's like the author's way of going "hey! That must be real uncomfortable for the protagonist so can we maybe stop!"