Did the osmonds have an older disabled sister?

No but they do have 2 older brothers, Tom and Virl who are totally deaf. They were both born deaf. However, on many occasions, they participated on many Osmonds shows both on TV and live events, the last being, as far as I am aware, on The 50th Anniversary Shows a few years back. From very young age, their parents taught them to play saxophone, tap dancing etc to enable them to learn communication through vibration and in turn they taught their younger siblings what they learnt.