I can empathise with you sooo much. I lost my sister to a car accident when she was only 18, and then, ten years later, my remaining sister lost her husband to another car accident. We (me and bro in law) didn't get on, but I know what you mean about bringing it all back. You think you may just be "getting over it" (but believe me, you never do) and then another RTC (road traffic collission) occurs, and all the awful feelings come flooding back.
I feel so bad for you atm, and can truly appreciate what you are going through. I can't offer any advice, other than to maybe have grief counselling if things get too tough.
I didn't go to my sister's funeral (as parents thought I was too young, I think - I was about 11/12 at the time), but that was so the wrong decision (dont blame my parents at all, they did what they thought was right at the time). So make sure you go to the funeral, say words / do a reading / poem / quote song lyrics, whatever, if you feel able to.
Hugs and kisses to you. Dont really know what else to say. We all handle grief in different ways, and you will, eventually, find your own way to get through this, but there is no definitive answer I'm afraid. xxxxxxxxxxxxx