At this moment, my thoughts about how to proceed are these:
Inform her prior to the visit that under no circumstances will we allow any physical contact to be initiated by her. If my kids, with their innocent loving hearts, want to give her a hug, I will allow that. But with her history of using a “hug” to threaten someone, and the fact that she dotes on male children and verbally attacks female children, I am not going to allow her to initiate anything.
I will also give her fair warning that if she makes any critical comments about myself, my husband, my children, my sister or niece or nephew, my step-dad (her ex-husband), or anyone else whom I know or care about, that the visit is over and we will leave. Similarly, if she tries to talk about personal matters or her therapy or our family history, we will leave.
Furthermore, I will inform her that she may not mention, critique or second-guess any of our boundaries, or we will leave.
I know that I cannot build a fence around her. My boundaries work only for myself and those under my own authority. But I am not going to make the effort to appear at the museum for a visit, unless I have given prior notice of my boundaries to her. At the very least, when we do have to leave early, I will not be kicking myself for not having given fair warning in advance, and I will be mentally prepared to follow through on exactly what I stated up front that I would do.
I already know that I have let her too far back into my life. My husband and I believe that both of us are still trying to wrap our brains around the idea that she is a dangerous person, despite all the years of evidence. His opinion is that now and then, until we prove it to ourselves, that we will reach out to touch the flame again and see if we get burnt. And that the only reason we are such slow learners in this matter is because this involves my mother, and not some other less-central figure in our lives. We had no compunction about cutting out some frenemies recently who behave in much the same manner.
So in preparation for our trip back home, we are going to create a list of boundaries for ourselves, and mentally prepare ourselves for having to defend those boundaries. Also, we will communicate our expectations to my mother. If she reacts badly, then we will simply cancel our visit. It would not be the first time we have gone back home and not seen her or some other family member who could not be trusted to not be abusive or manipulative.