How to evict someone in Virginia?

Send him a certified letter giving him notice to leave.  Talk to the district court in the area where the cabin is.  Ask about their landlord-tenant division, how to evict him.  If he hides from official mail service, then pay the sheriff's department to serve him your notice of eviction.

Since he has refused to comply with friendly requests to leave, you need to start a proper legal attack on him and his wife.  You evict them both.

It will involve an eviction notice and a proceeding in landlord-tenant division of the court.  Do it right and it will be over quickly.

First they get a notice to vacate. 30 day notice, in writing, document it (often mailed and also posted on the door). So say you give them until the end of February.

Then if they do not leave, you then evict.

She must legally evict the nephew. You have no control over his reaction when he is served eviction papers, but it's the only way to legally remove them.

Notice to leave in writing, sent by recorded delivery, if he doesn't leave by the notice period, then file in court for eviction, court will decide on eviction date and if he doesn't leave by eviction date, hire a court bailiff to physically remove him

Your mother is going to need to spend some money.  She needs a local lawyer.  Go through the local Bar Association to get a good referral.  Then she is going to need to be guided by him on the best way to do this which will probably be a 30 day notice pinned to the door.  She should, by the sound of things, expect that this will need to be a court ordered eviction and should budget accordingly.

Right before notice is posted, she might want to think about asking the lawyer to hire someone to take dated photographs.  As landlord she is entitled to access with notice.  This will give her a record of the condition both inside and out that she can use if he damages the place between receiving notice and the actual eviction.  Sometimes knowing there is a record will suffice to prevent damage.  Of course if there is damage, she can claim from him but nothing from nothing is still nothing so that may be pointless.

She should do this entirely through professionals.  If he contacts her, she should simply say that he needs to contact <name of lawyer>.  That way she is not drawn into drama.

Find him someplace else to live. Tell him he has to move there. If he still refuses, see a lawyer.