Probably the skeleton she performs the Black Sacrament on...

That wouldn't make any sense, why would she try to kill what is already dead?

She is not trying to kill him... He means that the skeleton that serves as a part of effigy to summon the brotherhood belongs to her husband. She is probably trying to kill business rivals.

It was Hemming. If you pay attention to some of his dialouge, he'll mention that Ingun and Sibbi are his children, not his siblings.

This is noted as a majotr inconsistency though, as he's also stated within the creation kit to be her son. I think it's probably best to take the Black-Briar family as what they are face value, with Hemming, Ingun and Sibbi as her children and her not having a husband. Otherwise you get into all sorts of confusing messes with incest and such.

Could be a small reference to Oedipus!

You can't deny the possibilty that that is exactly what this is though.

Maybe maven was trying to kill sabjorn, the onwer of Honningbrew meadery in whiterun...

And incest in the Black-Briar family would surprise anyone why?

What if, the assassin trying to kill the dragonborn was the result of one of her dark rituals? finding the letter may probably indicate that the Brotherhood failed. Since in my game that assassin came extremely early leaving me no chance to get to riften there is hardly a possibility to find the note before that assassination attemp.

Yet at this stage, I wonder how she could know the dagonborn at all, but probably its because she is affiliated to the empire and afraid the DB would go woth Ulfric. But then, she also already controls the city even with the stormcloaks being in charge.

I just can't remember anymore if that asssassin appeared before the event revealing to be a Dragonborn or after.