I made the mistake of explaining to a couple of freinds that I am in the process of getting diagnosed for Aspergers a little while ago. Although not strictly angry, their reaction made it very clear that they thought I had absolutely no reason to think this and that the whole thing was basically a stupid idea. It was odd - almost like they somehow took it personally or something.
I only became friends with them when I was about 19 at which time I was massively focused on trying to fit in, look more social and hide my weirdness. I suppose I can see why they might doubt it since most of my current traits are fairly well hidden most of the time. Still, I think it taught me a bit of a lesson about maybe keeping this more to myself since I do have a tendancy to want to overshare things like this sometimes (not that I am suggesting for a minute that you should keep this stuff from your sister).
I am really sorry that your family are not being more understanding. I think it is important to find people or just one person who you can feel comfortable talking to about this - even if it is just someone on a forum such as this. I think that many of us who start to suspect AS find it all very confusing and need validation where appropriate rather than flat dismissal. Feel free to pm my anytime if you do feel you need to talk to someone.
Si
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AQ46, EQ9, FQ20, SQ50
RAADS-R: 181 (Language: 9, Social: 97, Sensory/Motor: 37, Interests: 36)
Aspie Quiz: AS129, NT80
Alexithymia: 137