You know, I used to ask that question and get very angry at LE for blaming the family and NOT checking other leads. Then, sadly, I discovered that more often than not (anyone have stats?), someone very close/family IS to blame. Sad and disillusioning, but true. In this case I believe LE is right on and doing a great job. I've never, ever, seen such acting out from the entire family. There is something more than "wrong" here.
In your case, I can sympathize with you. The family is NOT always guilty and I feel for people like you, wrongly accused. Not meaning "you" in the subjective sense.
In my opinion, of course.
I shortened your post....
When LE ask family members to assist them. If family memebers aren't guilty most will offer ASAP to take a polygraph test...sometimes LE asks them if they would / other times the family offers. It saves LE alot of time in ruling members out. It also normally proves a family members guilt or innocence.
LE also begins there investigation with the family members who are closest to the victim. They can give the most insight.