Personality Disorders Support Group
Personality disorders form a class of mental disorders that are characterized by long-lasting rigid patterns of thought and behaviour. Personality disorders are seen by the American Psychiatric Association as an enduring pattern of inner experience and behavior that deviates markedly from the expectations of the culture of the individual who exhibits it.

I'm more interested in what your brother does when she does said behavior. Is he embarrassed? Does it appear normal to him? He seems to be enabling at this point. I think he should tell her that what she's doing is abnormal and she needs to get an MRI or EEG and if her brain's normal, she's got to then go into therapy for these anti-social behaviors. If he refuses, then he's got to start to see you without her. If he's too weak, then you either need to put up with SIL, or back off from your brother's odd situation.
Sippy cups and kids menu? Again... wow. This is a total jump, but could she have been sexually abused as a girl? Sometimes, our development gets frozen at the point of trauma. That's my only guess.
Obviously, this behavior needs to be addressed because if it's going to be over-looked, she's not welcome in my house! Her baby language, her constant talking about herself, her eating addiction is just ridiculous! We had two friends pop over on us on Mother's Day and they left after 15 minutes. They simply couldn't take her constant talking.. We were so embarrassed!!! We tried telling them how bad my SIL is but I don't think they believed us that it was to that degree until they witnessed it themselves.
I'm just at a loss at how to approach my brother about it. He's very well aware that she alienates everyone around her and he apologizes for her behind her back but yet again, he does nothing to stop it.... I guess after 30 years of being with her he's become accustomed to her. What makes me angry is that I think he just expects the rest of us to put up with her.
What she needs is to be psychologically evaluated and seek treatment but then again, in her eyes, she's above everyone else and thus doesn't require any treatment; she's simply above all that and not reproachable...
I agree with, it's one ODD relationship! I don't think she was abused in any way in childhood but I do know that her sister and mother are both the same way; always talking. I can't even imagine what a family get together would be like with the three of those cacklers in one room... Nightmare!!!!!!!!!!!!!!!!
But, I'm going to address this because it's not something that can be ignored any longer.. Thanks for responding.
Best, PD
Do an "intervention" with your brother with your sister. Just say that both of you ladies need for him to control his wife and he's got to put some conditions on your SIL.
Maybe he'll be on board and stage a REAL intervention with your SIL? I'd do the research and have some numbers for him to call when you sit him down. He needs to take action THEN, or you know it won't happen...