what are we supposed to say again when christians ask us where we get our morality?

[Esquilax]

Why is morality defined as the well-being of sentient beings? That’s completely arbitrary.

Not really, if you think about it: sentient beings, even under a theistic worldview, are the only beings capable of defining and identifying morality, and more importantly, they're the only ones capable of acting morally, and reacting to external stimuli. Without sentient beings you don't have any actors through which moral actions on either side of the spectrum can be performed, nor observed and identified.

What else is there? In a world devoid of sentient beings, does morality even exist? To be clear, you don't think so either, since (I think, feel free to correct me) you believe morality comes from god, who is a sentient being. That morality concerns the well being of sentient life is just a corollary of the fact that sentient beings are the only objects which morality, regardless of its source, can act upon.