You can change the gender of your sister's mother to male, and show her two fathers that way.
Other than that, there isn't much you can do. I would suggest attatching a blank box to your sister as a partner, name it something like 'X's BIOLOGICAL FAMILY FROM HERE' and grow her biological family from there. You could then add an explanation in Bio notes.
Currently there is no way to show adoption directly.
Hope this helps!