I have a PA younger sister who ticks all the boxes regarding PA behaviour - obstructive, forgetful (on purpose), unable to communicate directly, always appears to be sunny natured (unnaturally so), never gets angry (not healthy), blames others (especially me), behaves like a victim, procrastinates on tasks, doesn't take responsibility and rarely follows through if asked to do something.
It's been really difficult over the years trying to deal with her and despite what one person said on this forum that sisters are a gift, I would happily give that gift away, as it's not true.
My best advice is to accept that a PA's behaviour is unlikely to change (whether it's a co-worker or family member) and the relationship that you would like to have as a loving sister is not going to occur. No matter what you try to do, this will not happen. It's hard, but what you can do is try to make the best of the situation as it is - their behaviour prevents that close relationship that you would, I imagine like to have.
Therefore, for you to keep some semblance of sanity, accept her behaviour, keep a distance emotionally, stay on guard (it's hard) and as I read somewhere recently some people are best served loving from a distance.