Alright let's take this one step at a time.
Q: "i would able to go into past?"
A: Astral projection doesn't allow you to time travel. Time might seem slower or faster there, but that's essentially perception.
Q: "and the recent place where he is now?"
A: That's most certainly a religious question, you decide that for yourself. However, remains of his energy very well could be lingering on any of the closer planes.
Q: "can i meet him through astral projection??"
A: If he's there, he might not be.
Q: "and could i will be able to find the truth through A.P?"
- in regards to you suspecting your step mother killed him...
A: No, astral projection will not provide you the evidence needed to successfully determine if his death was the result of foul play. However, his death should be under investigation at least through an autopsy. Wait for the results and if it is determined he died because of foul a legal a new investigation will take place. During that time you should speak you queries to the police in charge of said investigation and why you believe what you believe about your step mother.