possible trigger....i agree with sabrina, there is no time limit on grief. especially under the circumstances you shared. i still miss my bgf from 20 years ago but as time has gone by i am better able to deal with the loss of her. we shared everything with each other until that day she didn't tell me what she planned to do. at first i was angry...how dare she!!!! but realized that sadly she was in such a dark place that she felt alone, all alone, even tho i could walk to her house, etc. had she called me, etc. that makes me the saddest...she "forgot", in her state of mind that i was there for her for all times..good and bad.
you asked if you could have stopped what your sister did....there are so many unanswered questions those of us have had losing a loved one this way. we will never really know the answer. i learned i had to get out of the debating society on my loss of her cause i just don't know the answers. i wish my friend had not felt so "alone". that is the hardest part for me to deal with but in reality that is the way she felt.
i'm sorry you were not notified sooner, etc. i don't know your family's reasoning on this. but glad you are talking about it here at pc. it won't take away your loss but i hope you will find some solace here.