When she asked him about siblings, he didn’t mention his brother because they have been fighting for ages.

When she asked him about siblings, he didn’t mention his brother because they have been fighting for ages. "Mention" needs a direct object. Similar verbs are: "describe", "explain", "suggest", "propose", "declare", "announce", "state", "express", "report", and "discuss". All these verbs feel awkward if they are followed by "about".

When she asked him about siblings, he didn’t mention his brother because they have been fighting for ages.