AITA for assuming I (31F) would be invited on a trip with my fiancé’s (35M) family?

My (31F) fiance (35M) and I moved from our home country to Europe for a 12 month sabbatical. His mum and sister are planning to visit us, which we are all excited about. Importantly, in addition to visiting us in our country of residence, they are planning a side trip to Greece to scatter a family member's ashes, which is very important to all three of them.  Due to the way the dates worked out, there are only ~9 days where both his sister and mum will overlap, so they obviously want to use that time to go to Greece (we will have time with each of them individually before and after that 9 day period)

I am really keen to travel to Greece with all three of them too, but I don’t AT ALL expect to be present for the ashes-scattering part of the trip - I never got to meet this family member, and it's obviously extremely personal. I therefore suggested to my fiancé that rather than come on the whole trip, I just join them for a quick 2-day stop in the middle (either before or after the scattering bit).

He proposed this to his family, and they essentially responded saying they weren't expecting that I would be on the trip and would prefer I don't come at all; they want to prioritise time for the three of them to bond and make their own memories. They will spend a couple of days in our country of residence before leaving for Greece, and his family are saying they want to make memories with me during those days, and leave the trip to be ‘just them’. My fiancé was extremely upset by this and now unfortunately there is conflict between them.

This really blindsided me. I respect (and have no desire to interfere with) their desire for alone time - I know my presence does change the dynamics. But I am really grappling to get my head around the fact that they aren’t happy for me to come just for 48 hours, to experience a new country together, and then they have the rest of the week without me. I am very happy to give them ‘alone time’ whilst I am there (e.g. entertain myself during the day, then meet up for dinner). From my perspective, this was an opportunity to build a memory and a fun new dynamic together as a new family; but it seems like they perceive it as a ‘cost’, which is just making me feel so sad and rejected. My fiancé and I have been together for 5 years and I really assumed that they saw me as part of the family, and that it went without saying that I was welcome to join.

I just need to know if I’m missing something here, because I feel like I’m going crazy. My fiancé and I are going to call them and talk it out (once everyone has calmed down) and I just want to soundcheck my feelings ahead of that. Am I being inconsiderate or projecting? Is it normal for them to want this as a kind of “last hurrah” as a family unit before my fiancé and I get married? Is there an angle related to scattering the ashes that I am overlooking? I have been turning it over in my mind and I just can’t understand. TIA