We know a mil.
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?