First off I want to apologize for my English as it is not my native language.
I am 20 years old and live with my parents and two younger siblings (11 and 8) and we have been dealing with this extremely difficult problem for a long time. My grandmother who is now 72 has always been extremely self-centered and has constantly been causing trouble ever since I can remember (and before that too from what I am told). Four years ago she started becoming even crazier and started doing some really disturbing things. It started with her putting her poop in her husband's clothes and shoes, hiding his glasses,spitting on her neighbor's car etc. and she was always very cunning about it, being careful never to get caught. He of course left her and my family felt sorry for her so she started coming for dinner every week. We didn't know then about all the things she had done to her husband because he didn't tell us about them until afterwards.
Then someday my mother saw that she had used one of the toothbrushes in our bathroom and confronted her and she denied having done it until she was pressed very hard. About two months later when I was walking to the gym I smelled a strange smell and found out that she had put vomit on the inside of my jacket. When I went home I found out that she had done the same thing to my mother's fur coat. My father then called her ex-husband and he told him that she had been doing disturbingly crazy things like that for about a year! He then called my grandmother and asked her about it and she denied it for a very long time before finally admitting it and asking us to forgive her. When he asked her why she had done it she said she couldn't answer.
This whole thing made me very shocked and I told my parents that in my opinion she should never ever be allowed in our home again. They didn't agree with that and she continued to come over about every two weeks. I have a very hard time being around her so I often went over to my other grandparents who live next door when she came over to avoid her. About a month after this incident with her putting vomit on my mother's and my clothes my grandfather's car had been scratched with some carkeys and about two weeks later it happened again. We soon found out that it had been my crazy grandmother who had done it because it had happened the last two times she was visiting our place. About two months later my father spotted that after she was leaving the toilet at our place she had some toilet paper with poop on it that she was going to put on something in our house! She is now in a mental institution.
This is all very disturbing but what I find even more disturbing is that we had been going to her place for dinner a few times the year before (we didn't really know she was that crazy then) and my 10 year old sister vomited after she came home at least three times after being there! So my guess is that she had been putting some poop or something in her food that made her vomit everytime she got home from her. It is at least very unlikely to have been a coincidence.
After hearing this I really freaked out and told my parents that she should NEVER EVER be allowed to visit us and never be around my younger siblings. They didn't agree and she is still coming over at least every two weeks. Although we guard her almost constantly and are very careful that she is never around the food. My parents still allow her to be alone in our bathroom which I am totally against. And she still comes over for birthday's and when the family is getting together and she is not constantly watched over so god knows what she could be doing!
This is making me very anxious and almost depressed and I am feeling very helpless because she has to come over every two weeks and I can do nothing about it. Believe me I have really tried to convince my parents that it is not right that she is coming over every 2 weeks. I could at most tolerate her 2-3 times a year or something and she would never be allowed to be alone.
I really need some help as to what I can do to convince my parents that it is wrong for her to be constantly coming over.