What should the younger daughter do to repent after taking money from her father's inheritance with her sister?

The girl must repent sincerely and increase her good deeds, as Allah the Almighty says: ﴿But whoever repents after his wrongdoing and mends his ways, then indeed, Allah will relent to him. Indeed, Allah is Forgiving and Merciful.﴾ She must also return what she took unjustly to its rightful owners. Furthermore, she should advise her sister to do the same, for "religion is advice."