When should I tell them I am pregnant?

Am I making the right choice? My fiancés family has been planning a reunion in Alaska for Christmas since January. My mother at that time was battling breast cancer and we didn't know if she would have another Christmas after this one so I said I was going to stay here with my family.  My mother has now lost her battle with cancer as of April the 10th. I told my fiancés family that I will be staying here to be with my father for Christmas since it will be the first one in 40 years that my mom will not be here.  They said I have 2 brothers  that can be here for my dad.  
A week after my mom passed I found out I am pregnant. Our last pregnancy end in January of 2016 with a stillborn. We my fiancés mom blamed me for the baby passing.  I don't want to tell them yet that I am 6 weeks pregnant but my fiancé wants to tell them so they do not book their trip to Alaska.  I want to wait till I am 12 weeks before we tell them but they want to get the tickets for the flight in the next couple days.  

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