A - Thanks for the question.
It is fine to ask family or friends who may be in heaven to pray for us. No prayer is wasted. The prayer will be effective unless it is to someone who is not one of the elect (those who will be in heaven eventually). Therefore, if the person is in purgatory or heaven it is a good thing to ask them to pray for us.
On the flip side, don't forget to pray for your deceased family either. They need your prayers if they are in purgatory.
The Catechism brings both ideas together very nicely:
958 Communion with the dead. "In full consciousness of this communion of the whole Mystical Body of Jesus Christ, the Church in its pilgrim members, from the very earliest days of the Christian religion, has honored with great respect the memory of the dead; and 'because it is a holy and a wholesome thought to pray for the dead that they may be loosed from their sins' she offers her suffrages for them." Our prayer for them is capable not only of helping them, but also of making their intercession for us effective.I hope this helps.
Other post you might be interested in:
**Can The Saints Hear Us?
**Where Did Purgatory Come From?