Ingrid

You recall that Muller sowed his bastard inside the younger sister, Ingrid which is interesting since the softness of her expression belies the wantonness of a woman who doesn't care for social appropriateness. She doesn't care, which is why she sleeps with her sister's husband behind the latter's back.

To settle the problem, you must ensure that Muller's bastard with Ingrid wouldn't affect the hierarchy of the family, and Susanne wouldn't feel threatened enough that she has to do something to take the child out of the picture. The easiest option that you can think off is to sign off Susanne's daughter as the legitimate heir.