My sister is taking things she knows has been bequeathed to others in Mom's will. Is this a form of contesting her will?

No. It's a form of stealing.

It's not a form of contesting the will - that's a legal procedure, in court, and after the death occurs. It may be a form of theft. But that would depend on whether your mother is okay with your sister taking the items. This is a murky thing. You mother's probably in no condition to argue over it, let alone take your sister to court. And your mother may be alright with your sister taking these things.

I doubt you have any recourse, anyway.

By all means speak to your sister about the bequests. In a perfect world you would point out to your sister that the items were meant to go to so and so, and she would agree to send the items on to them. In reality she feels entitled to those things or that the rightful recipients are not deserving enough, and WW3 will probably break out if you try to retrieve what she has stolen. Trying to appeal to your mom will either confuse her, hurt her or perhaps make her defend your sis, after all she is living with her. That's the way I see it, anyway.

She is only taking items she wants that were bequeathed to others. She only received a copy of Mom's will from the attorney that updated her will and family trust following my dad's death. Until then, she knew nothing about either and is now removing items that she knows are meant for others. Do I have any recourse to protect Mom's wishes prior to the progressive dementia?

What is your sister doing with the items that are named in the Will? Could it be she is putting them into a safe place so not to be damaged while your Mom is living with her?

I believe one can only contest a Will after the person had died and the Will has been read.