What is the interpretation of my fiancée and her sister seeing strange things in the house, what is the way to solve this tribulation that the family describes as a result of the grandmother reconciling with the jinn, and how can I deal with my fear of completing the marriage under these circumstances?

The sister can consult trustworthy specialists in ruqyah (definition) شرعية (Islamic exorcism/healing) and fortify herself with divine protections for safety from evil. It is advisable to dispel the phantom of fear and occupy her time with what is beneficial, and to avoid sitting alone and overthinking this matter. As for the questioner marrying her, there is no impediment if she possesses religious commitment and good character, along with seeking Allah's guidance (istikharah).