Nullifying a warlock pact

My party recently:

• Got to level 18
• obtained the swords of kas
• I have 17 levels in Bard and one in druid currently, because of that I can cast wish with the right feats, which I have currently.
• my character recently reunited with her last living family member, her sister, who is now a warlock.
• I want to nullify the deal, since the magic can't be taken away and having your soul gone sucks, but none of us know who the patron is.
• Could we learn who through divination spells? And if so:
• could a wish spell nullify the pact and un sell her soul? I would assume yes since the reality bending part is so ambiguous and encompassing