More than anything else, although it actually became a legal 'petite wrinkle' stuff, I think the letter was intended to reassure Christie's knowledge and feelings about her family roots. But, mainly, because at some point her cousin realized he had lost someone so fundamentally important in his life without knowing how he had neglected so badly, that he saw in her the only solid link to allow him reconstructing his past.
shareyes, i think he realised he had been unfair to her. i do wonder though why, if Max's uncle is supposed to have known he had a daughter, he wouldn't have made a will to leave the house to her anyway. it struck me on thinking it over that it is a bit unlikely he would just write to max telling him he wanted Christie to have the house, rather than making a proper will in her favour. i would imagine the people who had bought the house from Max would have put up more of a fight.
shareI saw the letter mainly as the means to negate the sale and retain Uncle Henry's legacy. But it also was an act of kindness and fairness towards Christie which reflected the change he was under going.
The funny thing is every time I've watched the film before I'd understood he would share the ownership of the house but when I watched it this time I realised it was now his cousins. Which changes the meaning of the letter.