Since your In-lw do not have to worry about school zone, they can be anywhere in NoVA. The primary issue, I think, would be how much can they tolerate the traffic from VA to DC. Meaning, parts of NoVa would be challenging to go thru the traffic during busy rush-hour.
Also, what parts of DC do InLws has to travel to?
NW? or farther away from downtown, say, Tenleytown or even farther out?
Also, are they looking for SFH or condo?