After having seen the film over 10 times now, (oh boy! tomorrow I will go and watch it on a HUGE IMAX screen!), I can say that I have never doubted in the least that Jake's mother knew...
From the very first viewing of this wonderful movie, it was plain-as-day for me to see that she knew... This is what made that last scene of Ennis' visit to Jack's parents so poignant and utterly heart-wrenching.
Her eyes reveal that she knew...her miniscule gestures and the trace of tears in her clear, open eyes show us that she knows what Ennis meant to her son Jack...and regardless of what her husband says...she was accepting of it, and ok with it...because she loved her son Jack so much...
But, sadly she is caught up in a relationship where the man rules the house...the man speaks...while the wife stays dutifully silent...
Though using what little she has, she makes her (personal) standpoint clear to Ennis...
The coffee and cherry cake offering, saying that she had purposely kept Jack's room exactly as it was when he was little, saying to Ennis that he is welcome to go up to the room, the gentle hand on his shoulder, the knowing look in her eyes while she hurries to get Ennis a paper sack for the two shirts when he comes down from Jack's room (he doesn't have to say a word...she knows exactly what they mean to him!), turning her back towards her husband in a steadfast and almost belligerent stance while looking directly up to Ennis' face and asking him in a steady and controlled manner, to come visit them again...
And most of all, in the doorway, as Ennis takes his leave...out of direct eyesight of her dominant husband..."in private with Ennis"...she accepts Ennis' whispered "much obliged ma'am" ...with a sad look which makes clear to the audience, and to Ennis also, that she knew...
A tender moment which lasts but a second, but which is laden with so much emotion, that it would make even the most stoic amongst us choke back a quiet sob...
sam