Actually ADD does tend to run in families, as does bipolar disorder.
In my experience, there is no real stigma anymore to ADD....just about everyone knows or is related to a child who has it.
Bipolar disorder is less widely understood, but it is becoming a more common diagnosis. Again, many people will know someone else with that diagnosis.
I think any "stigma" either of these carry would be less than the stigma of a work history that shows a lot of job changes, etc.
The fact that your sister has ADD makes it much more likely that you also could have it. Don't let your life be chaotic just out of fear of what someone will say. The meds make a huge difference...you'll be so much more productive and happy with yourself.
You deserve a happy life, with stability and achievement. If you need medications to achieve that, then so be it.
There is no shame in having an illness. Do you love your sister any less now that you know she has ADD? I doubt it. Be as kind to yourself as you are to her, and get yourself the health care you need to thrive.