I agree that it is mainly symptoms, circumstance, and ability to cope in the given moment that change. Perception is also important, I think. If I were diagnosed in early childhood, classic autism would have seemed obvious (speech delay, lots of typical stims, etc.). However, I was diagnosed after many years of learning to cope and "fit in", so I appeared more on the Asperger's side of things. I still have the same problems I did as a kid, am prone to shut-down, selective mutism, and need at least some assistance to function on a regular basis. And yet, I can fake it well enough and for long enough to often pass as just a very odd NT.
Something as traumatic as losing a sibling could certainly cause a big increase in symptoms. It doesn't even take anything serious for me to see a difference. For instance, I can go to the grocery alone and get out without causing too much trouble.... but if I have the extra responsibility of bringing a child with me, the task will have me overloaded and headed toward a meltdown! This applies on a larger scale, as well. Any big change in "my world" lessens my overall ability to function normally.
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ASD mama; ASD four-year-old; hilariously questionable one-year old.