coffin, tung tree
櫬 centers on a wooden container for the dead: a coffin. The tung tree meaning is a separate botanical application, likely from the same character used for the tree's wood.
櫬 combines 木 (tree, wood) with 親, which likely contributes the sound. The character originally referred to a type of wood (tung tree) and later came to mean a wooden coffin.
The left side is 木 (tree/wood), and the right side is 親 (parent, intimate). Picture a wooden coffin made from a tung tree, intimately holding a loved one.
For シン, imagine a shin (shinbone) resting inside the wooden coffin: shin -> シン, and the coffin holds the remains.