Just pointing out, that miniseries is non-canon. There is so much in it that contradicts mainline X-Men books - including events that kept being referenced after its publication. So who the hell cares what Brevoort says about it? In canon books, his grandmother was implied to be Monet. Plus, Bishop and Storm have nearly ended up in a relationship multiple times as recently as Age of X-Man - with her possibly being his grandmother never being a reason for it being nixed.
You seem to want her to be his grandmother for some reason. But at the end of the day, it was never explicitly said so she isn't. Marjorie Liu and Christopher Yost wanted Laura Kinney/X-23 to be a lesbian and I believe wrote her that way. But it was never confirmed and she has been shown explicitly heterosexual since then. It's not the best comparison, but the point is, in X-Men things need to be made clear to work.
And yes, I know X-Men has had parental incest before what with Legion reality warping himself into his father banging his mom and Madelyne Pryor maybe raping a teenage Nate Grey but it really doesn't need this unless Bishop is going to be a heavy handed analogy for Severian and Storm for Dorcas.
Plus, Bishop had tons of tests run on him. You think Beast would've found out years ago they were related unless he secretly wanted them to bang cause he's a degenerate.
Point is, they're not related. They could be eventually, but they're not right now.