Now that you mention it, I also struggled with that to begin with. My penguin edition had a dramatis personae list at the start that I ended up referring to a few times. Rare for me to use them. So, a crib sheet if your edition doesn't.

There's a sequence with the boys out on the town which helped me cement each of the main male protagonists images in my head. Fairly early on I think. Pierre + Andrei being main characters, Nikolai (the younger) and Antole being the rest of that group.

I also I ended up classifying the characters into three generations, the young men/woman, the older parents, and the younger children.