Deely,
welcome!
The DSM IV for Antisocial Personality Disorder:
A. There is a pervasive pattern of disregard for and violation of the rights of others occurring since age 15 years, as indicated by three (or more) of the following:
(1) Failure to conform to social norms with respect to lawful behaviours as indicated by repeatedly performing acts that are grounds for arrest.
(2) Deceitfulness, as indicated by repeated lying, use of aliases, or conning others for personal profit or pleasure.
(3) Impulsivity or failure to plan ahead
(4) Irritability and aggressiveness, as indicated by repeated physical fights or assaults.
(5) Reckless disregard for safety of self or others.
(6) Consistent irresponsibility, as indicated by repeated failure to sustain consistent work behaviour or honour financial obligations.
(7) Lack or remorse, as indicated by being indifferent to or rationalising having hurt, mistreated, or stolen from another.
B. The individual is at least age 18 years.
C. There is evidence of Conduct Disorder with onset before age 15 years.
D. The occurrence of antisocial behaviour is not exclusively during the course of Schizophrenia or a Manic Episode
Not all sociopaths are psychopaths and vice versa. The DSM IV focuses on the behaviour and not on the personality traits, such as charm, glibness, and lack of affect, conscience and empathy, etc, that Robert Hare's Psychopath descriptor encompasses.
This nephew's firestarting is dangerous so the primary issue you have is ensuring your physical safety, i.e. get rid of the firestarter.
Since you describe that you are now living on the same property as your mother-in-law and husband's nephew, and that you have invested in the property, is it possible that you and your husband have some ownership of the property?
If so as owners you can throw this nephew off your property.
If the property is owned by your mother-in-law, is there a possibility that you or your husband could get a power of attorney over your mother-in-law so that you can report your nephew's theft of her effects and money, eject him from her property and file complaints with the police and eventually get a restraining order if he returns and persists in firestarting, stealing and cheating your mother-in-law?
If there are no grounds for a power of attorney (because your mother-in-law is mentally well) then try to empower her with information to facilitate her ridding herself of her abusive grandson (press charges herself, eject him, and get a restraining / exclusion order if he persists).
Also you could look into and implement protecting her resources from her grandson, and without anything to gain, he might leave of his own accord.
Of course, Grandmothers tend to look at even rogue grandsons with rose tinted glasses, and your husband's nephew seems to be the type to shamelessly exploit such endearment. If this is your circumstance, past taking all precautions against the nephew's pyromaniac tendency, I can only suggest you do everything you can to limit your emotional involvement with your mother-in-law's situation, given she is then abetting him.
Personally I would not antagonise anyone who has a lack of respect of others property, is deficient of empathy and is a firestarter, especially if they shared my abode!
I hope there is a fast resolution to this extremely worrying situation for you Deely.
KT