Recommendation letter from supervisor in this case? During most of my undergrad I have spent a lot of time working in a research center where my mother is director.

During this time I worked with her and many of the researchers there.

Of course, I cannot ask for a recommendation from my mother because we are related. Would it be OK to ask for a recommendation from one of the other researchers on the center I worked with?

To clarify, this is a public center and while my mother is higher in hierarchy than them, she is an employee too, as she don't own the center.
You can definitely ask a recommendation letter to other researchers. Try to get the most experienced of the ones your worked with to write it, if you did a good job, that should be no problem.