As the holy verse in the Quran says God's mercy encompasses all. You can ask the Almighty to show His mercy upon them, and overlook their shortcomings, etc.
Unfortunately some Muslim groups believe it is haram to prayfor a non-Muslim in any way, but reality is such an approach is difficult for one whose family might not have recognised Islam and be good people.
Just requesting Almighty God to show His mercy would hopefully assist them in their next life.
And Allah knows best.