Perhaps use safety pins at or near corner, to indicate size (ie, 1 for king, 2 for for queen, etc), for all parts of set.

If all bed sheets are the same color, wash sets separately. Assign a permanent marker pen color for each size or bedroom, and mark either top 2 or bottom 2 elastic corners of fitted sheet (same location for mark, different color for sizes or bedroom). Pillowcases get washed same time as sheets. You can handstitch an X in colored thread but bleach may ruin it.

Not on point but, on behalf of a lifelong friend who survived 8 yrs after AIDS diagnosis in 1993, there are indirect deaths related to being HIV+ or diagnosed with AIDS: Treatment and protocols were/are costly and time consuming (continuous multiple doctor appointments with multiple specialists, then adhering to multiple regimens) so people died because they could not afford to get adequate and timely treatment; protease inhibitor side effects could kill you (cholesterol levels in the high 3 digits/low 4 digits = pulmonary embolisms); and early onset dementia (not sure if from HIV or AIDS), affected critical decision making (stopped heeding medical advice and adhering to treatment, began/increased self-medicating).