Is the hijab obligatory in front of one's mother's second maternal uncles, given that one has lived with them since childhood?

We do not know what you mean by your maternal uncles from your second mother. If you mean the brothers of your father’s wife, or the brothers of another foreign woman who raised you, then these are non-mahram men from whom you must observe hijab. As for your maternal uncles: the brothers of your mother and the brothers of your grandmother, they are mahrams, and you are not required to observe hijab from them.