Ashley,
This is an extremely tough situation I am in a similar situation as your fiance and do not have a lot of contact with my family so speaking from the other side all you can do is be there for your fiance and let him heal and mend the relationship on his own terms. If you force a relationship it won't really be genuine. He needs to heal from the pain in his time and all you can do is be cordial to his family and let them know that while you hold no ill-will towards them, it is contingent on your fiance's willingness to have a relationship. I'm sure the history behind is extremely traumatic and he just needs time. You could encourage him to talk to you or someone about it to help the healing process for his sake. It seems like his mom is remorseful for the past and ideally they can repair their relationship but until he is ready to forgive, there is not much that can really be done. I'm sorry you feel caught in the middle, you shouldn't have to take on that burden but again I'm sure there's a lot of unresolved issues. I hope this is helpful to shed some light on the the other perspective and speaking from experience, time sometimes does a have a way of helping things work out. As for the Aunt and Cousin situation, maybe when you guys get back yall can grab a brunch or something together and share the memories from the trip.