I have an aunt exactly like that! Luckily I don't have to deal with her very often anymore, but as a child I never left that house feeling good... and now that I'm grown I see just how miserable she is with herself. She has fewer and fewer friends, and less and less in her life that satisfies her. I just got off the phone to my cousin (her daughter who turned out fine BTW) and we traded thoughts about it. We both wish she were a happier person, both for our sakes and for her own, but she isn't and probably never will be, the point is, WE are happy people and cheereful overall, we have good lives overall, and this woman raised 2 great kids (adults now) for which I'm grateful to her for, I'm glad they're in my life.
She's miserable, you aren't! *smiles* Try to let your 'ears' glaze over when she gets going, and then when you realize she's paused for breath, say something like "Oh I'm sorry, I missed that last bit, I was thinking about a)a recipe, b) what time to set the table, c) where I put my X, Y, or Z." It will let her know whatever she was bi... er, complaining about went right past you.
I see lots of good ideas here, but liked the one about asking your husband to really "LISTEN" to what she says. I suspect he's just so used to her tone and attitude, he tunes it out completely. He may still not "Get it" though... some otherwise perfectly nice men never do when it comes to their mothers.
I hope she's not staying long... ((hugs))