Lisa,
You have definitely been through a very terrible tragedy - especially when she was in your care and for the police to have to come to your house to tell you - then you having to tell your sister - I cannot even imagine it -I will keep you and your family in my prayers.
My son was hit by a car while walking (the car was going about 55 mph) and I can remember how I felt when we got the call - they didn't think he would make it through the night - he was in a coma for 6 weeks - he has a traumatic brain injury and we will have to take care of him the rest of his life (he is 26, now).
I cannot imagine what it would have felt like to have lost him - I was on this forum when we got the call - it was the day after Cathe had her second baby and I was reading all the lovely posts about her little boy - I will never forget it.
We can forever ask why - but we do not know - 17 is so young - we always wish we could go back to that day and change it but we can't - that's why it is called an accident.
Keep praying for the strength to make it through this difficult and trying time - Hugs and kisses to you and your children and your sister.
In His love,
Cheryl