Couple of thoughts on this:
I've half-way considered poor uncle Harold a means to tie the boys to the outside world, reminding them (and us) that the real world off the mountain is waiting, - the world they are soon going to have to live in. A place where they have family and family-related obligations. A place where people are ill and even dying. A place where supporting the family, being part of a family, illnesses and births and deaths and the implications of such events take on a much greater importance relative to the emotions the boys are grappling with on the mountain. Yes; a far cry from their isolated heaven on the Mountain.
Aguirre being the messenger between those two worlds just seems to fit, somehow.
I've pondered how little we get to see and hear about the familiy members. Especially about Ennis's family, perhaps. There are his brother and his sister, both of whom are married, so I assume as time goes by there are nephews and nieces as well. Before he's divorced, there are the in-laws, Alma's sister, her mother (and father and grandmother) - probably further family members on Alma's side. Through their connection to his girls Alma's family in a way remain his family even after the divorce. Yet we never get to meet all these people (apart from a tiny glimpse in the wedding - I suppose that's Ennis's brother and Alma's sister we see left and right of bride and groom?) We never get to know them. Judging from the film narrative, they meant little in Ennis's grown-up life. Ennis must have kept in contact, though - there must have been Christmases, Thanksgivings, christenings, weddings, other events and just family meet-ups.
Apparently, apart from the untimely death of his parents, none of that made a significant impression in his life, neither as one-time specific watershed events, nor as representative of the direction his life is going in. While many of the meetings with Jack did take on such importance. I suppose it's all showing us how much family took back stage in Ennis's mind to his relationship with Jack.
Nevertheless, when seeing Jack's uncle Harold in that light, he's useful simply as an early reminder that the boys *have* family connections and obligations, beyond what we get to see in the film itself.
Perhaps he's also an indication that family ties and obligations are stronger and mean more on the Twist side of affairs than on the Del Mar side - also considering Jack's going back to help his folks out in later years.
Perhaps the total picture of family relations in the film is showing us that Ennis, having experienced the shock and trauma of his parents' death, consciously or subconsciously made sure to keep family at arms length thereafter - simply to avoid another such painful grief.
Perhaps uncle Harold *not* dying after all contrasts the Del Mars' too-early death - or rather contrasts the impact on Ennis and Jack, respectively: Of losing family members - or of keeping them. Of experiencing the grief of abandonment through death, or of having it pass you by......
Lots of thoughts here, brought on by the elusive uncle Harold!
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I've somehow gotten the impression that Aguirre knows Jack's mother. I just think he speaks relatively respectfully there - as if he knows who Jack's ma *is*, and feels especially obliged to do her a service and convey a message himself after having talked with her. It's not entirely impossible that they knew each other once upon a time, is it? As we later learn, Jack's mother *is* a woman who inspires respect and kindness even in quite gruff guys (unless they're her hubby). Aguirre knowing Mrs. Twist from before might be one reason why Jack was hired on the first year. Might even be the reason why Aguirre holds back from letting Ennis and Jack have it after he discovers them
in flagranti.