Grisella

You recall that Muller sowed his bastard inside the youngest sister, Grisella which is amazing since he, out of all men, managed to seduce and bed her. Then again, being young, perhaps Grisella isn't thinking much when she allows Muller to ravish her body.

To settle the problem, you must ensure that Muller's bastard with Grisella 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.