I personally would not respond to this piece of emotional manipulation from your sister also because she wants a response. A response to such disordered of thinking people is the reward.
People from dysfunctional families end up playing roles. Your role here is likely to be one of scapegoat. Your dad seems very much to be his wife's enabler (and secondary abuser) and your sister is the golden child (a role itself not without price but she remains unaware of this).
I would not let your dad at all off the hook either; he has singularly failed here to protect you from his wife's excesses of behaviour (out of self preservation and want of a quiet life. He is truly a weak man).
And as for her comment re what's going to happen when we don't have mum and dad anymore, well I would advise you not to go down that rabbit hole because its a trail that leads nowhere.
You may find the following from the website called "emerging from broken" helpful:-
"Your parents have had their whole lives to make a positive difference when it came to you. They made their choices, and apparently through the grid of how these type of statements are meant to be taken, my parents choices are acceptable but your choice NOT to put up with abusive and disrespectful disregarding treatment is NOT acceptable? It’s like people are so brainwashed by this whole thing that they don’t even realize how stupid it sounds to be told to accept abuse/neglect/disrespect just because ‘they’ are ‘family".
"That is what this “what if your parents die” question is about. It is about parental rights and entitlement ~ something that YOU as their child don’t have in a dysfunctional family system. People are so afraid that if they ‘hear you’ and validate your reasons for not having relationship with your parents, or for going no contact, that they might have to think about the dysfunctional relationships they have with their parents or even worse, with their grown kids.
It is indeed not possible to have a relationship with a narcissist.
You may also want to have a look at the current May 2020 onwards "well we took you to Stately Homes" thread on these Relationships pages.