Is what the sister is suffering from, in terms of marital problems and her reaching the point of potential divorce, considered a divine punishment for the uncle's injustice towards his children from his first wife, knowing that the father does not provide for them due to his poor financial situation?

No one is held accountable for the sins of another, as Allah Almighty said: "And no bearer of burdens will bear the burden of another." And due to the statement of the Messenger of Allah, peace and blessings be upon him, during the Farewell Pilgrimage: "Behold, no offender offends but against himself. A father does not offend against his child, nor does a child offend against his father." Therefore, what befalls your sister has no relation to your uncle's injustice towards his children.