For someone sensitive to knee/joint issues, I highly suggest a step-through frame. You also likely want an upright riding style. Check on bikes like the aforementioned Trek Verve, the Liv Flourish/Sedona, and the Specialized Alibi/Crossroads/Roll. My wife rides an older Trek step-through (a 7100 WSD) and I must say that I enjoy it also...being able to step right through the bike.