My relative can be nice, but like her sister (who I married) they have zero tolerance to anything said that may sound remotely like criticism. They both become extremely aggressive. I must state that they are twins and their mother has exactly the same tendencies.
For example my wife and I were staying at her sisters house and the "change filter" light came on the AC. So I bought one and did it. When the sister found out instead of thanking me, she went red faced and became angry as if I has just broken her best dish.
From being passive she shouted comments like "it didn't need changing","I wait for weeks after that light comes on","it won't make any difference" she also said she did not believe us and that it wasn't dirty, even though she saw the change filter light flashing.