To use a sippy cup without my parents knowing I used to just hide the Sippy cup in my closet behind some clothes.
Then I would just wash it out in the bathroom when I needed to wash it.
The same was true with baby bottles I had to wash when living with my parents.
I would wash them in the bathroom sink to clean them.
Then I would hide the bottles and sippy cups in my closet.
You can put the bottles inside your closet behind some clothes.
Just make sure your parents do not go through your clothes and find it though.
Or hide the bottles or sippy cups in a computer case if you have a desktop computer.