Did the Mirror of Erised show what Harry's family actually looked like?

Question

In Harry Potter and the Philosopher's Stone, Harry looks into the Mirror of Erised and identifies the people he sees within as his family.

She was a very pretty woman. She had dark red hair and her eyes — her eyes are just like mine, Harry thought. […] The tall, thin, black-haired man standing next to her put his arm around her. He wore glasses, and his hair was very untidy. It stuck up at the back, just as Harry's did. […]

And slowly, Harry looked into the faces of the other people in the mirror, and saw other pairs of green eyes like his, other noses like his, eve a little old man who looked as though he had Harry's knobbly knees — Harry was looking at his family, for the first time in his life.

Did the Mirror of Erised show what his family actually looked like, or merely what Harry imagined that they would look like? Harry had already learned a bit about his mother and father, but not about his other relatives (such as the knobbly kneed one), so it's unclear to me if the Mirror or Harry is filling in the gaps.

Answer 1

Yes, it showed what his parents actually looked like, at least.

Before looking in the Mirror of Erised, Harry couldn’t remember what his parents had looked like, and had never seen them before. He only started dreaming them after looking in the mirror (though it’s possible he remembered them somewhere subconsciously). Before that, he couldn’t remember them at all, hadn’t seen any pictures of them, and had no idea what they could have looked like.

“He couldn’t remember his parents at all. His aunt and uncle never spoke about them, and of course he was forbidden to ask questions. There were no photographs of them in the house.”
_{^{- Harry Potter and the Philosopher's Stone, Chapter 2 (The Vanishing Glass)}}

However, when he’s described, Harry’s father looked exactly like he did in the Mirror of Erised.

“Excitement exploded in the pit of his stomach: it was as though he was looking at himself but with deliberate mistakes. James’s eyes were hazel, his nose was slightly longer than Harry’s and there was no scar on his forehead, but they had the same thin face, same mouth, same eyebrows; James’s hair stuck up at the back exactly as Harry’s did, his hands could have been Harry’s and Harry could tell that, when James stood up, they would be within an inch of each other in height.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 28 (Snape’s Worst Memory)}}

Harry’s mother also looked like she did when he saw her in the Mirror.

“It was one of the girls from the lake edge. She had thick, dark red hair that fell to her shoulders, and startlingly green almond-shaped eyes – Harry’s eyes. Harry’s mother.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 28 (Snape’s Worst Memory)}}

No one else in Harry’s family is ever described, so we can’t compare anyone else to how they looked in the Mirror of Erised, but it did show Harry’s parents how they actually looked without him consciously remembering them.

Answer 2

The power of the Mirror doesn't appear to be limited by the knowledge of the viewer.

Had Harry ever actually seen what the Philosopher's Stone looked like, before he saw it in the Mirror? He knew what that he wanted to find it, but not use it - but the Mirror showed his reflection holding it, before it presented him with the stone itself.