I once saw a quote that said something along the lines of If you don't know how to treat a person with a disability watch their sibling. If the person with a disability has a good relationship with their sibling, I think this is a good rule of thumb. It also can be applied to family members and friends. At times just by observation, you can grasp a better understanding of how to interact with someone living with a disability. Sometimes the outside world or society sees a person with a disability just as disabled. On the other hand, a sibling may carry the sentiment of Oh, that's just my sister (in a positive connotation).
In addition, I think this notion could be beneficial for those with disabilities when interacting with others who have different disabilities. Observing the interactions of their loved ones can provide some guidance for ways you may want to interact In the future. Be sure to use good judgment. People have varying comfort levels. Some may be more open than others and that's okay.