Learn Spanish (Spain): ¿Que si se lo he mencionado al yerno? - Are you asking if I have mentioned it to the son-in-law?

This is an echo question, which is the focus of this lesson. When someone asks you a yes/no question and you want to confirm what they just asked before answering, you repeat their question starting with que si. It is the conversational equivalent of saying 'Are you asking me if...?'

If the sentence already says al yerno (to the son-in-law), why do we also need se at the beginning?

This is called indirect object doubling. Even though al yerno explicitly tells us who is receiving the action, Spanish strongly prefers (and often requires) keeping the indirect object pronoun (se in this case) before the verb alongside it.

Wait, the indirect object pronoun for 'to him' is le. Why is it se lo instead of le lo?

You are right that 'to him' is normally le. However, Spanish has a strict rule: whenever le or les comes immediately before an object pronoun starting with L (like lo, la, los, or las), it changes to se. This happens simply to avoid the awkward sound of le lo.

Could I use the simple past here and say mencioné instead of he mencionado?