Sister was right in front of our mom, who was looking at sis with a dead stare. Then mom jumped as she suddenly saw her. What happened?

Those are diagnoses you were given.
Now what you need is neuro-psyc testing with actual tests to test cognitive levels to decide what safety levels are.
This isn't called attempts at diagnosis failing, but rather not following up with the testing you need in order to assess her safety and ability to function on her own (or not).
You need neuro-psyc eval; get referral from her MD for cognitive testing.

As to this jumping who knows. She has dementia almost certainly or an overactive anxiety and startle reflex? Who could know such a thing? Your guess is as good as ours, and in fact better as you know her overall reactions on a daily basis and we can have no idea of that.

Watch on you tube, the senator, Mitch McConnell. When he went blank a few times for reporters, think it was during the summer. I'm sure it's easy to find. See if your mom stair looked like his.

Everyone is guessing. No one can possibly know what happened. Make an appointment with a medical professional to find out if you have a need to be concerned.

I agree with funkygrandma, if you hate caregiving and aren’t able to handle the situation then that’s another issue for you to address.

Like Funkygrandma says, your mother needs someone who will be her advocate to see that she receives care.

Wishing you and your mother all the best.

Startle reflex can become noticeable after stroke.

To me it sounds like your mom may being having some type of seizure, but like already mentioned, we here on this forum can only guess. So if you're really that concerned about this issue put a call in to her doctor, or message them via moms patient portal.

And on a side note, your profile says that you "hate" being your moms caregiver, so I do hope and pray that you are not her full-time caregiver, as everyone deserves someone to care for them that doesn't hate what they're doing.
I know that caregiving is hard, trust me I know all too well, and not everyone is cut out to be one, so I hope your time with your mom is very limited at this point and that she has others looking after her that don't hate it quite as much.

I am with Willie. Where are Moms aneurysms, in the brain? If they are getting larger, they could be pressing on parts of the brain. Enlarging means the arteries or vessels linings are getting thinner and will eventually burst causing death if on a artery.

I would call Moms doctor. I would also getbher out of independent living and into an AL.

Possibly an absence seizure or a TIA, but that's just me guessing.

I'm wondering if your mom was napping, with her eyes open. Just a thought

There is no way that anyone can answer this question.

We are not medical professionals. People can guess about what happened. That’s about all.

You need to speak to your mom’s doctor about these questions.

Wishing you all the best.