To be honest with you, even though it sounds a good way to raise awareness and avoid people touching you, there is, unfortunately, the possibility that it might turn out to be a reverse psychology technique; there may be some awkward people in your family who would actually be provoked to touch you even more if they were to read the message on the T-shirt, just to see what happens. It's like a red button saying "DO NOT PRESS" above it in bold lettering. It makes some people more tempted to press it. Or someone might just see you from the back, unable to read the message, and touch you.
You shouldn't necessarily need to wear specific clothing in order to tell people you don't want to be touched. The T-shirt might attract unwanted attention, making things even more awkward.
You might want to tell your parents how you feel - you're very concerned about being touched at the gathering, and you'd like them to forward this onto other family members.
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I'm sailing across Spectrum Sea, in my little boat.
The waters of the port were choppy. After I set off, there was a long, massive storm.
Years later, however, the sea calmed. I'm still on tranquil sea, but I'll never reach the Neurotypical Beach.