I can sympathize with your brother in law, though an hour and a half sounds an awful ordeal and a very long time to access a fistula. But, without knowing exactly what his fistula looks like, its actually quite hard to answer your question.
The vein in most fistulae is quite close under the skin and often is tethered there (as you rightly note) by quite a lot of thickened scar tissue from previous needling. While massage has the potential to increase the suppleness of the scar tissue in the subcutaneous (underneath the surface of the skin) space between the skin and the vein wall, it may also - if handled too energetically - cause damage to the vein wall ... especially if the recent puncture site(s) have not yet fully healed and blood is allowed or encouraged by manipulation of the puncture site to leak/seep out from the vessel into the tissues.
A fistula, used at least 3 time weekly, does need a little time to heal - post-dialysis - and early 'manipulation' might promote a re-bleed or blood leak. Yet, in under 48 hrs, he's got to get back into it again, for another set of needlings. So, I guess what I am saying is, too early and there's risk .. but, too late, and its the next dialysis.
I wonder if he might be a good candidate for the creation of a buttonhole. We use buttonholes in the majority of our home patients .. probably in more than 30 of our home HD patients .. and, increasingly, in both our nurse-needled and self-needled facility patients. Yes ... we actively encourage and train patients to self-needle in our facilities as well as at home ... especially for those who, for some reason, can't or won't consider home but would be capable of home self-care if they would train for it.
Buttons are also useful when fistula 'ladder' access is difficult .. perhaps the case with here ... and, once a button is made, it works well and sustainedly in most patients - allowing needling with great ease and without pain. Clealry, a button should be made according to the buttonhole guidelines: same nurse, same site, same angle, same track, same everything ... and by a nurse skilled at fashioning a good button. indeed, as the button-making process switches from sharp to blunt needles, this would be the ideal time for the patient-hands-on-transfer of needling care from nurse to brother-in-law.
While subcutaneous tissue conditioning through massage or gentle manipulation may have a role to play in keeping an AVF 'nice' and 'supple' .. it is something that would need to be done with great care and with direct knowledge and supervision of his renal team. This is not to call into question your clear skills in this area ... but the renal team should be involved and know that this process is happening.
For me, however, deciding and advising on this without intimate knowledge of the state of his access would be dangerous indeed!
He should put this question to his team.
He should at least discuss the potential for buttonholing with his team.
He is a lucky guy to have you by his side to help, where and however you can.
That is the best I can offer, from afar .. but I hope it may have been of some help.