。
She knows your mother well.
Literal
She [topic-は] your honored-mother [object-を] well is-knowing.
お母さん with the honorific お~ prefix is the standard polite/respectful term for someone else's mother — even in casual speech, you'd more naturally say 君のお母さん than 君の母 when referring to your listener's mother. Speaking about your own mother to outsiders, you'd typically use plain 母 — Japanese marks the in-group/out-group distinction (uchi/soto) lexically: humble inward, respectful outward. This sentence does both — slightly casual 君 paired with respectful お母さん.