your first priority is your father's safety. you can talk to social services or you can go talk to the police and tell them your suspicions. I don't mean to alarm you even further but there are some things that might help to confirm or dismiss your suspicions. First of all, you need to know if his wife is mentioned in your father's will, if she has taken any life insurance out on his name. secondly, the most common used poisons are heavy metal based poisons. after a while there will be white lines on his nails and a while after that his hair will start to fall our in big chunks. as I understand you can't see your father now but maybe you can ask a close family member that does see him and that could keep the secret. if he ends up in the hospital, tell the doctors right away about your suspicions as poisonings are not very common and so they will look for some disease to figure out what is wrong with him.
after you settle this part straight you need to be able to prove that she is harassing you in any way, tapes, witnesses, letters, anything. you could sue them but first make sure your father is alright.
As far as requesting an autopsy goes if she does not agree you will most likely need to take her to court.
I strongly advise you to seek out a lawyer and have everything prepared.