I'm not sure it's called for to compare GP to a "Holocaust denier". This Wikipedia article is informative:

https://en.wikipedia.org/wiki/Deaths_due_to_the_Chernobyl_di...

According to the article, there are 60 agreed-upon long term deaths from Chernobyl. But there is a debate about how to measure long-term deaths, with some claiming figures into the tens of thousands. The nuances of the debate are covered in the article.

So I think GP's point is fair, because even at the highest figure of 60,000, that is far lower than the millions per year from fossil fuels. This article estimates that fossil fuels cause 1 in 5 deaths worldwide:

https://en.wikipedia.org/wiki/Fossil_fuel#Illness_and_deaths

That doesn't take away from the tragedy that you and your family experienced, which is something that should always be considered in these debates.