Quote Originally Posted by tohca View Post
Мы знаем свекровь.
Does it mean "We know your mil" or does it mean "We know your mil" or "We know a mil"?
We know a mil.

Quote Originally Posted by tohca View Post
Is this a proper / usual sentence?
No. In this sentence, it is not clear who they talk about. I don't can even imagine a situation where this phrase would be suitable.

- Мы знаем твою свекровь. (OK!)
- Сочувствую вам. / I sympathize with you.

- Мы знаем свекровь. (Bad!)
- Чью свекровь? / Whose mil is she?