It’s not obstruction of justice, but the parents should be still be called-out for trying to deprive the public of a full investigation.
Called out, yes, absolutely, but it’s not obstruction of justice.
Now, if they actively tried to stop any investigation of the case, that would qualify.
Roger Stone tends to be a little over the top sometimes.
Maybe they've been told they can join their son if they don't try to squash it....