sialise:

Stanford is the twin’s great uncle. Meaning he’s the brother of a hypothetical grandmother or grandfather. That’s what great uncle means. So the existence of a sibling is proven right there.

What proves it’s a male, is the name Pines. The only way for Dip and Mab to have the same last name as…

THANK YOU! Thank you for bringing that up, I’m with you, I sure that Stan has a brother for those exact same reasons, and people lose their minds on YouTube especially, claiming “Stanly” isn’t real, and is just  theory. Kinda ridiculous considering all that you just pointed out and I wish more people would talk about things like that, because its basically irrefutable evidence.  
One can say “Stanly” is only a theory, but no one can deny at this point that Stan has a brother. Do we know his name? No. Do we know if he was Stan’s twin? No. But he know he exists, or at least existed.

As for whether or not he will play any significance, well, as someone who’s been writing for years, and has goals to become a tv show animator, I have to question the use of having Stan being the twins’ great uncle, if the sibling (in this case a brother) played no significance to the story at all, especially since I believe Stan was based off of Alex’s grandfather not great uncle. Just seems like it would over complicate things if you ask me.
Course, about 95% of the fandom was convinced McGucket was the author and that theory ended up not being true, so who knows with this man lol

sorry if there’s any bad typos,  just got a new laptop and I’m not used to the keyboard yet

P.S.