ah, don't you sometimes wish YOU could be the screw up and everybody would run themselves ragged fixing everything FOR you? i can be stranded on the side of the road in a snowstorm with my car on fire and still not get any help! LOL
but seriously - your sister is a grown woman. she is not the only person who lost a loved one. at some point that "excuse" no longer works. it could be that your dad sees his helping as a good, loving, caring thing and maybe trying to do what he thinks your mom would have wanted??? taking care of her babies?? i don't know, but it's hard to fault the DESIRE to help.
whereas you saw that any money you sent was not fixing a damn thing, he has yet to draw that conclusion. so he'll get enabling until he runs out of resources, health, or patience.
you can't your sister. you can't change your dad.
life is full of tough choices and not being part of a dysfunctional family system is a BIGGIE. you are allowed to make those choices, regardless of what the others still mired in the dysfunction might think. it won't be easy, but sticking to your boundaries and no longer serving as a RESOURCE to someone who is messing up their life is in YOUR best interest.