ANSWER:

If the parents had repented and warned them against the sin while they were alive, it’s only the children who commit that sin would be sinful and not the parents. 

If both mom & dad introduced & encouraged sons & daughters to do haram deeds due to their ignorance but later they didn’t do this continuously for becoming religious or other cause, but sons & daughters still continues to do it in various form,still dead mom & dad will be punished in grave or sons & daughters are one & only sinners?

Sins | August 23, 2019 2:54 am
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Ibn Majah hadith 4245 It was narrated from Thawban that the Prophet ﷺ said: “I certainly know people of my nation who will come on the Day of Resurrection with good deeds like the mountains of Tihamah, but Allah will make them like scattered dust.” Is this hadith referring to masturbation or something else?
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If a person drinks alcohol,his prayer is not accepted for 40 days.That means his prayer is valid but not rewarded.Does that 40 days start from his day of drinking or does it start from when he starts to pray.Ex:If I’m drinking today,and I start to pray 5 days later,then will my period of 40 days start from the 5th day or from my 1st day of drinking itself.At which day my prayer will be rewarded?