I saw bits of it a couple of years ago... but it's not that old.

Brother and sister travel by boat to a fishing village. Brother is sick and sister is a stunning blonde. Sister spends most of the time talking to people on the island trying to find a cure for brother. Brother spends most of his time in bed.

Towards the end there is a surprising incest scene, and it turns out the brother is related to some weird underground critters. I think the brother ends up living (or dying) underground with the weird semi-human critters.

Does anyone recognise this movie, it's been driving me nuts!