peeing in the bathroom sink!

Hi!!
We have four cats, the newest editions to our cat family has been two seven week old kittens, who have taken up residence in our upstairs bedroom for a few weeks. Recently one of our cats has begun peeing in the bathroom sink! We've tried to deter her by keeping the doors closed, but we caught her peeing on a chair! I understand that it's confusing for her because there are suddenly two new cats in the house, and her litter box has been moved from upstairs to downstairs, however even when she had her box upstairs she still used the one downstairs, so we figured it would be a natural change for her, plus she still defecates in it, but she refuses to pee in it! Why is this, can someone please help?

Please take you kitty to the vet. Statistics show that when a cat pees on a smooth surface it is usually health related and a urinary tract infection. The cat needs to go on antibiotics to clear it up. Until you take her in, and I hope it is soon, keep a little bit of water in your sink, and a litter pan in the bathroom nearby.

Thanks!