Mistress

Life, is what has to be,
I know that today,
When all I have is misery.
My love, my life, where you are?
I can not declare,
But thats whats most bizarre.
You say you love me in every way.
Along with my sister,
But Im your mistress,
Because shes your wife.
What should I do?
My only option is to take the knife.