Red Harlow isn't Uncle because he is roughly the same age as John Marston in the games, Red Harlow was born like between 1860 to 1870 and Red Dead Revolver takes place around 1880's likely in 1888 and so they couldn't be the same person at all.

R* has a very odd stance on the canonicity of Revolver. They say that it takes place in its own universe, but there are several references to Red Harlow in Red Dead Redemption 1 & 2.

I always took it as the events of Revolver the we saw it isn't canon to the Redemption series. The basic events happen, like Red getting revenge for his father and the Battle Royal in Brimestone, but it's a tale that has been exaggerated through word of mouth and Dime Novels.

But anyway, as it was stated, Uncle cannot be Red. Uncle is, at minimum, 50 in 1899. The events of Red Dead Revolver happen sometime in the mid - late 1880's. Uncle would've been around Mid-Late Thirties to Early Forties during the events of Revolver, even though Red appeared to be around his Early-Mid Twenties. The ages simply don't match up.

When Red Harlow is mentioned by characters in the Redemption games, they’re basically referencing fiction. The story of Red Harlow and his exploits would be myth in the Redemption universe in the same way that Arthur and John are fiction in our world, so the fact that he’s mentioned at all makes no difference. Plus, at the end of the day, it’s just Rockstar referencing one of their games, which is fair enough.