What I would do is pass the end of the hose in the window to one of my children, or if I am alone at home, just pass the hose in the window so that it does not fall out, and making sure that the end nozzle is tightly turned off so that no water can flow from it.

I then turn the hose on. Go inside, and then once inside I put the end of the hose in the plughole and twist the nozzle until a strong stream of water comes out and the flow of water pushes the obstructions down the pipes. I would also leave the water on for a while just to make sure that whatever was the obstruction was well washed away with force and plenty of water.

Normally the problem is hair.
If you have sons and daughters with long hair, this can be a very big problem as the hair gets washed down the plughole.

I have known people’s daughters (told to me by their parents) that they brush their hair while under the water and the result is a loss of hair down the plughole far more than if they had not brushed their hair.

Sinks:

These ideas can work in the Kitchen Sink. But you can always undo the “trap” below the sick and clean that out.
On one occasion I did that and found peas blocking the pipes and asked the woman how they got there and she said that when there were extra peas after dinner she would push them with running water down the plughole.

So do please be sure to ask the people who operate the sink about their habits to help you get an idea about what is going on and then offer them advice to limit pipes blocking.

I hope you fund this useful.

All the best from
James M Sandbrook.
14-10-2021.