Sisters are a gift - sometimes a crazy one, I grant you!
Is there any way to "hate the sin, not the sinner"?
Her cheating isn't the sum total of what she is. Maybe her advice to you stems from her envy (not the bad kind) of your long marriage.
I hate for any sister to "write off" another sister. Could you agree to have this discussion not be a part of your relationship? But leave the door open for other things? "Noone can make you feel inferior without your concent." Eleanor Roosevelt
I will not be vanquished. Rose Kennedy