My mom keeps asking about her deceased family. How should I respond?

Have you told her that the person she is asking about has died? If so what was her reaction?
If she was upset then you do not tell her that any more friends or family have died.
"Susie" is at the store"
"Aunt Mary went to work today"
"Bill can't visit today, he has a cold"
Then after you have said something like that say something to acknowledge the person.
"Aunt Mary has the prettiest hair, a lot like yours, lets fix your hair"
Then begin a task that will take focus to something else.

When I made up stories about the dead relatives being busy or away on vacation, my mother with advanced dementia would get really mad and say, "YOU'RE FULL OF SH*T". She had a very sharp tongue.

When I told her they'd all died, she'd say, "YOU'RE FULL OF SH*T AND HIDING THEM ALL IN THE CLOSETS!"

Ativan worked rather well to calm mother's agitation.

Take out some old pictures and let her tell you stories of what they did in the good ol' days.
When my daddy got really ill he tore a lot of the pictures out of the photo albums and would tell me stories.
They live in the past and they want to be there again!

Fibbing is a good thing in this case. Just tell her something comforting, such as sister is on vacation, and change the subject. Mom will be asking the same question over and over no matter what you say. So why increase her stress and anxiety? (Your stress and anxiety over doing it is another thing entirely.)

Also, it's the kindest way to deal with it. I'm always in favor of that.

Is she asking to see them? Or just talking about them in general? Either way, I don’t think that I would bring up their death to her.

Some people say to be honest and tell them that they are dead. Others say that it will only cause them to grieve over and over again because they won’t remember what you told them.

I didn’t go through this with my mom. I’m sure that this is an unpleasant experience for you to deal with. I’m sorry that you are going through this.