Why is my sons medical bills being divided by everyone insurance in my household (sisters and parents )when he was in my car at time of accident?

If I am understanding correctly, your son was a passenger in your car and lives with you.  I am presuming that your son is not a named insured on any auto policy.  Other people live in  your household who own other cars that have insurance.  The way PIP works is that anyone who is a family member in the household who has auto insurance would also be responsible for his bills so they share the costs amongst them.  Their premiums should not be effected by this and the $15,000 is probably the liability coverage and has nothing to do with PIP (Personal Injury protection----this is the NO FAULT portion of the policy which means no matter who's fault an accident is, your own insurance, if you are  a named insured, or those in the household, if you do not have your own policy, pay your bills. ) usually, the traditional policy has $250,000 in coverage for PIP coverage.