Hi MaternityLeave
Was the party at MILs house? Why did she have control of the date (fitting it around appointments??)
Ignoring the cancer issue for a minute, if the party was at MILs house then that might partially explain the assumption that they would cut the cake. They seem to struggle to consider others viewpoints/wishes , probably exacerbated by their stress with cancer.
Putting your acceptance as an act of kindness is fine, and generous of you. But I am sure you are also strong enough to not allow the cancer to become a total excuse for getting away with things that are really put of order As Summelove says, you can never get this time with your son back.
I totally understand why you would want to be the one to cut the cake etc. If it is any consolation, as the years go by with ones children, so many lovely milestones come and go that one experiences with them and so missing one becomes somehow less important.
Keep in touch