I think its cause their brother is still in their lives so any money he gives to them he'll take , and his family wasn't dirt poor before the brother took the money.
So might as well do some investing so he can give more later
it was mentioned that his uncle died in a specific date but later when mc woke up as him but it was still quite early from the announcement of his death, so he is taking his time although he knows that his Mom might be struggling he might be readying or make some arrangements before approaching,
who knows ╮(￣▽￣)╭ this is just me theory
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His (original uncle's) family were fairing quite well since the hometown friend from the first few chapters, cared for mc's family together with his own at the same time with the money MC initially gave him. After 4 months, mc would soon visit his family and they'll have a feast. He has his reasons as to why he didn't immediately went to his family as soon as he reincarnated. He is quite anxious on what to do in his new life for his family to not suffer anymore, and that mindset made him postpone visiting again and again for four months. The earlier comments were true, he is just preparing things for his family since he has a quite big ambition for them to live comfortably and happily.
Did he give his mom's family some money yet or nah??? I feel like he forgot his original purpose