A lighthouse(?) with candles that send you back in time

I read a book a few years ago that followed a teenage boy, and I think one of his siblings, as they moved to their grandpa's house on this tiny island. I think the grandpa owned a lighthouse because there was one on the cover. The grandpa had a collection of handmade candles that, when lit, sent the person burning it back in time to a specific moment. The boy discovered this without his grandpa knowing, I think, and they had a bit of an argument over it at some point. There was one candle grandpa kept on the fireplace mantle that was almost completely burnt down, and the boy eventually got too curious and lit it, sending him back to a night many years ago inside a cave at sea level. I forget the exact moment it depicted but it was something sentimentally important to the grandpa because it involved someone dead. Maybe the night he proposed to his wife? I don't know. Toward the end of the book I remember it mentioning that there were three magic things on the island that could only be found by one person, the boy of course, and I specifically remember one of the magic things being a unicorn or pegasus that was on the cover of the second book in the series (which I unfortunately did not read...).