First and foremost, of course, is that if they're not worried about it being totally GBOT-compatible, that's their right. I can't "make" anybody fit in.
That being said, this came up somewhere else a year or two back. I agree that trying to shoehorn in a nephew into Egon's backstory is problematic; I had to push things quite a bit to make Jen Spengler work.
I really wouldn't want to retcon in another Egon sibling; and it would be up to Jen and Ed if they wanted to make their characters brother and sister (I don't get an impression they'd really like the idea, though I could be wrong)
If it were really up to me, though, I'd make Ed a Spengler cousin: he's a descendant of the Lewistown branch of the family, as established in the animated episode "If I Were A Witch Man", which in GBOT is the ancestry of Katharine Melton, Egon's Mom (a different branch of the family stayed in Poland until the 1930's, when Edison and Cyrus's parents emigrated to America, per the unused GB2 backstory.); Spengler was the maiden name of Katharine's mother, so I'd suggest saying that Ed's paternal grandfather was Katharine's first cousin; Ed's father would be Egon's second cousin, and thus Ed would technically be Egon's second cousin once removed (and a third cousin of the Spengler Twins). He could still call Egon "Uncle" informally, though.
Just my two...or maybe ten...cents on that.
