(and she is older than yustaf if she was younger than him it means the duke married a single women and gave birth to the fl hence make them siblings)
Also the only person related to both ML and FL is there younger brother(dead) who was born after Duke and the 2nd duchess marriage... make him the only one related to them by blood.
.... married (and she is older than yustaf...
It matters if she was born before yustaf that means it was before the time duke married the 2nd duchess or unless duke had a long time affair with her while being married to the 1st Duchess
I see many ppl getting digust by both ML and FL like showing a sibling's love relationship (Incest). No! Stop it! Get some help... its it's not like that let me explain.
So basically ML's father the duke married *Twice* the first wife gave birth to yustaf and when she died he married again the 2nd wife who was either divorced or widowed and already had the FL from her first marriage.