It's nobody related to Ish and it's not Sam's grave.
How do you know it's not related to Ish? I mean, it could be. I haven't heard anybody from ND say otherwise, but then again they haven't said it is related to Ish. But I agree that it's definitely not Sam's grave, because they both look like it's the first time they've seen the grave and why the hell would they carry a corpse across the country just so they can bury it, right?
Yes, I agree that it is not Sam's grave because Ellie said "I forgot to leave that stupid robot on his grave." - She would not say that because the grave was right in front of her, lol. Either that or my English is doing tricks on me again. And I am pretty sure they'd bury Sam/Henry together or next to each other because they were a family.
I have acquired the grade of Master in The Last of Us story/character/game knowledge as I have played it through 9 times.