Mom is peeing in a container in her room and now in the kitchen. Last night she dumped her pee in the kitchen sink.

My nieces still talk about catching MawMaw with the large soft butter container she was using to pee in. No dementia here. Her bladder would empty the minute she stood up. Best solution was to catch it before she had a mess to clean up. In her case the toilet was a few steps away but not close enough. Since your mom was already relieved, the next place your mom might have wanted to be was the kitchen. Just guessing here. I think we all can become desensitized to things we are more familiar with.
Get your mom to an urologist to see what can be done to help her with this problem she has successfully taken care of until she got “caught”. A urologist was able to help my mom but not before she had an accident at church and was so humiliated she would never return.
Okay I admit the fact that she thought the kitchen sink was okay is a problem. Tackle one problem at the time. See if she can be helped controlling her bladder when she first wakes which might be different from why she can handle it later in the day as you described.

I’d use this situation as an opportunity to see where her head and judgment really are. So, let her know in very certain terms that pouring urine in the kitchen is unacceptable and place a bedside commode by her bed that you or the two of you empty in the bathroom. Then, see if she cooperates or not. If not, you know that something in her judgment is not working. She may be unable to understand her actions. When I inquired why my LO was having odd toileting issues, health care providers told me that she was lazy and spoiled, but later She was diagnosed with dementia. Imo, doctors don’t know that much about early signs that something is off. Poor judgment is often a sign, even before memory issues surface. In my case, my LO said that she could not go to the bathroom. She had no explanation. Her legs worked and she could do what she wanted, but for this she had no explanation. She eventually started back using the toilet, though, for a while. About a year later, she became unable to care for herself. She was then diagnosed with significant dementia.

If your mom is thinking clearly, then you should be able to resolve this issue pretty well.

Get her a commode and empty it for her every morning.

Emptying pee into the kitchen sink is revolting, I'm with you there! - but bleach the sink, as many times as sets your mind at rest, and then forget about it.

Urine is basically sterile when it leaves the human body (or should be! - have you checked her for a uti, come to think of it?) so while revolting this stunt of mother's is eminently survivable :)

Re-reading though. This is odd.

She has an en suite bathroom, no problem getting to that during the night, is there? Or is there? Where does she keep this night-time container, by her bed?

And what was she doing in the kitchen before she got "caught short" today? And what sort of container?

The "can't make it all the way to the bathroom" is clearly just an excuse; but the trouble is you need to know what question it is that she is afraid of answering with the real reason. There could be some child-like sudden fear of something in the house, some pain or difficulty, some obsession about body fluids. Maybe she doesn't want to sound silly. Maybe it's something embarrassing. Try to talk to her about it separately from the standard domestic hygiene issue! There could be an explanation that then suggests a straightforward solution.