Well considering I have http://skyrim.nexusmods.com/downloads/file.php?id=2710 that makes Ingun awesome...screw the Black-Briars! They're literally the worst of the whole "1%" people. But considering I'm the Listener of the Dark Brotherhood, Maven wouldn't mind. In fact, I'd think she'd be happy that her son-in-law is the Dovahkiin and Listener. Puts her in a higher position naturally.
That is, until I lead my rebellion across Skyrim and butcher her for sitting on a bench I want to sit on.
The problem with with the mod is that I don't like the idea that Ingun turning into a vampire... unless if I'm missing something here. I don't necessary have a problem with the Black-Briars. Maven is a strong businesswoman who does her things to survive in this harsh world. Same with Erikur in solitude who eventually agrees to become allies with the Thieves guild once you do his special job. Who wants to live their lives being poor? Having to degrade yourself in public as a begger, and for what? for a lousy septim or 2? We all have weaknesses. Someone like Maven fears of poverty. That's why she will go out of her way to get rid of her competition. Can't fault the woman for trying to survive.
As a RP stand of point you're right. It's a proven fact that Maven craves for strong contacts therefor the Dovahkin (who has connections not only in the thieves guild, but also the college of winterhold, Dark brotherhood and the companions) would be a great addition to her family, since she can brag about it, and use it for her advantage, if anything happens.