Jenny is Jamie's sister, and we'd know that even if they didn't share a name since she's just as fiery as he is. We learn that she once kneed Jack Randall in the stones, and when Jamie returns after years of hiding, she greets him by yelling at him. Yep—they're definitely related.
We don't get to spend too much time relaxing with Jenny at Lallybroch, the Fraser family estate. Not that there's any relaxing to do there anyway, since as the family matriarch, Jenny is in charge of all the grounds, and her husband Ian, who is one-legged, but loyal and hard-working, helps her out with keeping the tenants in line, raising children, and growing crops. Busy, busy.
In the short time Claire and Jamie spend there, Jenny does give Claire her stamp of approval. She trusts Claire to rescue Jamie after he is captured by the British, and since she has kids, she gives Claire a good idea of what it's like to be pregnant.