Weiss and Ruby are both autistic, but Ruby rarely masks and even when she does try masking she doesn't really know how to go about it - partly because she's never really had to - while Weiss spent most of her childhood being forced to mask, absolutely hates doing it, and finds it exhausting, but unfortunately it's a force of habit for her at this point
After a while, Weiss gets comfortable enough around Ruby that when it's just the two of them, she does stop masking. Usually when that happens, Weiss is already exhausted from masking all day up to that point so she just sort of slumps next to Ruby and wordlessly bumps against her as a way of requesting Girlfriend Cuddles
Related to this: Ruby actually took a relatively long time to start talking (somewhat supported by canon's inconsistensies about how old she was when Summer left) and while she's pretty talkative now she does still have the odd nonverbal moment. Weiss, on the other hand, was raised by Jacques, who definitely did some fucked-up shit to make her start talking at a "normal" age - but now that she doesn't have to deal with him anymore, Weiss does tend to go nonverbal as soon as her mask drops
(Side note, this headcanon does give Weiss something in common with Neo, which definitely has some kind of storytelling potential)
Also - neither of them is actually diagnosed. In Ruby's case, Yang and - once he became more present - Tai sort of figured it out by doing their own research, and in Weiss's case she just had no idea about anything related to autism until Ruby mentioned it, and took longer to realize she might have it herself