The mind can be a very odd thing.
I'm reading a book and the real detective in the book said the worst evidence is an eye witness-- there can be ten eye witnesses and there will be ten varying stories... some similar and some VERY different, yet they were all at the same place while the crime happened.... why so many views? because each mind takes past, present and expected future and molds the memory from all that.

Maybe the guilt you feel is powering your memory...?? Guilt can be so powerful... it can keep one stuck for a lifetime.. I know that so well.

have you ever talked to your sister about your memory of her trauma? maybe asking her to clear some things up for you, would be helpful.
I recently talked to the sister that was in the car with me when our mother tried to kill herself(she tried to jump out while on the highway)-- I thought I was the only one that remembered it-- but to my surprise, she remembered it all just as I had! it was a relief to hear her speak of similar memories, was like a validation.
can you try to talk to your sister? it's hard but, maybe she would do it if you voiced to her how important it would be for the both of you.

anyway-- that's all for now.... I have to think some more.....

best to you,

fins