From El Capitan OS forward iMovie 9 started to become increasingly more dysfunctional. With iMovie 8 it probably is even worse. So, I suspect that your difficulties with iMovie 8 likely relate to it becoming increasing dysfunctional with the newer operating system. Unfortunately there is no way to upgrade iMovie 8 at this time, since it is a discontinued application.
In case the above is not the cause of the problem, try opening iMovie 8 in your Mac's guest account.
You can also try booting up in safe mode. See this link for instructions.
https://support.apple.com/en-us/HT201262
You can also try resetting the System Management Controller (but no longer available for the new M1 computers):
https://support.apple.com/en-us/HT201295
As a last ditch workaround, you can try making a slide show memorial video in the iPhoto or Photos apps, with both videos and still images if you like, and then export it to your desktop as a .mov file. You will be able to add music and transitions and Ken Burns effect to the slide show, and a title. Not quite as comprehensive as iMovie 8, but still produces a very nice product.
Sorry to hear of your sister's passing.
-- Rich