Big flashing halogen content warning for both the film and the book. Campion's Sweetie often represents disability in a hurtful and damaging way, and is very explicit in doing so. And Triomf? If there is anything you should not read, do not read Triomf.
Triomf is set in apartheid South Africa in the suburb that was built on top of what had been Sofiatown. The novel was written in Afrikaans, which I don't read; I've only read the English translation. Sweetie is set in Australia.
Both of these texts tell the story of involuted white families with a disabled family member. In Triomf, the disabled character is the biological result of the family's complete rejection of outsiders, and is presented as a sort of personification or emanation of collective disgrace. In Sweetie, the family's turning-in and co-dependence is much milder than in Triomf, but is still an issue: the main protagonist, a non-disabled woman, can't quite escape the grip of her childhood home due to the failure of her family to respond appropriately to her sister's mental illness.
For comparisons, you could also look at Campion's An Angel at my Table (whose protagonist has a passage through a psychiatric hospital) and maybe some of her later films (which I haven't seen). I haven't read anything else by van Niekerk, so no suggestions there. Faulkner's The Sound and the Fury could get thrown into the mix.