Answered by Sayyidi Habib Umar bin Hafiz (may Allah protect him and benefit us by him)
How do we give advice to parents or relatives who are older than us if they disobey Allah?
We should ask elders or people they respect to give them advice. We can also give them books or show them videos which we think may affect them. The consequences in the next life of acts of disobedience may be mentioned to them but indirectly and with the appropriate respect and courtesy.
In general we should not be overly affected by the negative things we see in them. We should focus on their positive attributes and do what we can to rectify the situation.