My mom is 86 with dementia and is constantly crying out for dead family members. In particular, her mother and sister. Why is this?

Because she is now living in her long term memories and her brain is permanently broken, that's why.
Might be time for you to educate yourself more about this horrific disease of dementia so you will be better prepared for what is yet to come and so you won't be caught off guard.
The book The 36 Hour Day is a great place to start, along with all the many videos that Teepa Snow(a dementia expert)has on YouTube.

(((hugs))) It broke my heart when my mom started calling for her mother.... I think it comes down to them having lost their sense of time and place plus feeling helpless and vulnerable the way they did as a child, all we can do is try to offer love and comfort.

Basically you need to be grounded in the understanding that her brain is "broken" and cannot computer reality versus ideation and hallucination.
Oliver Sacks said, and I repeat here all the time, that they have a very real world when they have dementia, but it isn't OUR world.
We can't understand it. They can't explain it.
And there's not a lot more to say about it.