jail time

Would I serve jail time if convicted to a second felony of drug possession, hydrocone. The first felony being a Class E felony/theft charge.

You very likely could yes...it is up to the DA and judge how that would be handled. have counsel hired asap!

my sister called the DA and told them that she thought that I had a mental illness that has never been diagnosed. Will that hurt my case or help it. What happens with that. Do they do a competency evaluation ordered by the court. Will I still do jail time if they do diagnose me with a mental illness or will I do impatient somewhere with probation after that.

The DA makes their own interpretation...they don't normally rely on incoming phone calls..but get your lawyer to argue any point that medically may help you etc.