"sink onto" is a valid phrase in written English.
It is used to indicate a gradual settling (of a person or object) in a particular place. For example, you could say, "She sank onto the sofa and closed her eyes."
He could remember water bouncing off the bottom of the sink onto his stomach and chest.
They sink onto the sand, still faintly warm, radiating a lunar glow.
Occasionally they sink onto the floor, but to collapse or rest, not to dance; they move while kneeling, and soon they're upright again.
After turning off the magnetic field, the microswimmer will gradually sink onto the substrate due to gravitational force.
It observed the importance the characters give to freedom, citing the scene where Taloche becomes concerned that water is being "held against its will" in the taps, and "releases" it to overflow the sink onto the bathroom floor, and then the stairs, with Taloche blissfully sliding down the stairs as if he were on a Disney ride.
Once the gates meet in the middle, they fill with water and sink onto a concrete pad, effectively blocking any storm surge.
I sank onto my living room sofa.
His moment of grappling with the world had passed, and his chin sank onto his chest.
Signs wilted as cold, tired protesters sank onto the wooden benches to wait for their trains.
One by one, the great beasts sank onto their haunches on the sodden carpet.
In the early morning, at the Woodfield Mall outside of Chicago, Roberto Melendez sank onto a bench and rubbed his eyes.
