1) If we ask Allah for forgiveness of our late parents, can their kabira sins be forgiven?

2) Is there any specific Amal or Dua for this?

1) With Allah anything can be forgiven

2) Any Duas that sincerely come from the heart is the best of Duas. And to repeat the following verse would be best:

Holy Quran 17:24
وَاخْفِضْ لَهُمَا جَنَاحَ الذُّلِّ مِنَ الرَّحْمَةِ وَقُل رَّبِّ ارْحَمْهُمَا كَمَا رَبَّيَانِي صَغِيرًا

And lower to them the wing of humility out of mercy and say, "My Lord, have mercy upon them as they brought me up [when I was] small."