Dealing with Family Members with Problems?

I am looking for serious advice in how to help my 15-year-old deal with some family members with diagnosed mental disease. A member of my family was diagnosed with Borderline Personality Disorder with Depression and affective instability by a psychiatrist (long story there I won't bore you) and she and her husband are in complete denial. Other members of the family have a lot of denial going on as well, and I have for many years limited time spent with them to as little as possible. I have only allowed one or two public gatherings such as a dinner with many family members in a public restaurant per year. I have invited them to certain public gatherings involving my children such as my older daughter's graduation in a public football stadium. They always fail to show up for my kids' activities then pull them aside next family gathering and tell them nonsense like they were afraid to come for fear I might still be mad at them even though at all public meetings with them I am polite and civil and have never shown any form of anger or hatefulness to them. Friend who know the entire story often marvel how I can treat them as if there is nothing wrong when we are at these gatherings, completely without my asking or commenting. Now they are pressuring my daughter to go spend time with them near the diagnosed person by staying with her daughter a few doors down for a week. I feel it would be unsafe to allow this so said no. They were texting my daughter convincing her I was "just mad because of some dust ups" that happened between this woman and myself (The so-called dust ups are related to her ultimately being diagnosed and almost resulted in severe harm to one of my children and myself). I have blocked them from texting her phone. I have tried to explain to my daughter although yes, I was angry at the time of the incident, my refusal to allow her to spend a week in their home is entirely out of concern for her mental health and physical safety. This woman is very devious and I want to prepare my daughter to withstand her in the future. Is there anything further I should be doing?