Once again : Uncle, Rachel and Susan are in a 3 way relationship. Uncle understanding that Rachel has urges that he cannot satisfy, pushes Rachel in MC's arms. And also it's a way to test him further. So when Uncle dies, Rachel is still in relationship with Susan, she is not 'free'. And selfily Uncle might think than no man can replace him in Rachel's heart.
The most important thing for Uncle is that there is an heir to whom pass along the achievements he made so far. The odds to produce an heir are much higher with a 20 years old girl than with a 40 years old. And in the long run, the odds that MC stays in a united family with a girl his age are higher than with a spouse 20 years older. When time takes its toll MC might seek for a life that his ageing wife can no longer live. So once again, better for the Uncle to push MC in the arms of a younger one.
Last, Uncle cares for his daughters and he knows that MC will do his utmost to care for his wife. With Emily and Chloe in the race, there is a good chance that at least one of the girls will be well taken care of.
That's for the rational about Rachel not being one of the choices.
Then you can write a totally different story where MC fucks what people think and fly away with Rachel, you can't fight Love