Bear in mind that ethics and morality are opinionated, and perspective based. Personally, I'd say that if something elicits compassion it is therefore worthy of moral significance to me. To me, it's not just consciousness but rather what arises from consciousness that matters. To say something is deserving of compassion is relative to the suffering and misfortune of an entity. Suffering and being able to perceive misfortune comes from being conscious and having a degree of subjectivity. So if something isn’t able to suffer or be aware of its own misfortune, it is not particularly rational to feel compassion for it.
Empathy could be how one dictates moral significance, but I personally find it has its drawbacks when it comes to unconscious entities. The feeling of empathy, as defined by the Webster dictionary is the imaginative projection of a subjective state into an object so that the object appears to be infused with it. We humans and many other animals are predisposed to and will likely feel empathy for things that have relatable characteristics. The problem with using empathy to dictate a course of moral judgment is that characteristics such as self-propelled movement, vision, hearing, etc. could be said to have moral significance and anything that does harm to anything with those characteristics could be said to be immoral when those are things that modern-day robots and computers can even do. Sure, one can certainly hold the opinion that causing harm to those things is immoral, but I think it would be unlikely that when push comes to shove one would attribute moral consideration to something like a laptop or even cellular life. You could pose the questions of whether life is deserving of rights or if doing harm to life should be deemed immoral. Homeostasis, organization, metabolism, growth, adaptation, response to stimuli, reproduction, all of these things I personally can't imagine most people genuinely or seriously care about enough to attribute moral significance in terms of doing harm to them, for example, cutting down trees, mowing grass, cutting your hair, having organs and tissue disposed of after surgery, just to name a few. I would think that the one thing most people probably care about and can agree on is that if an entity currently possesses the capability of suffering and perceptive misfortune it then should be given moral significance.
When it comes to the question of "If you know something has the potential to be sentient is it unethical to intentionally prevent that from happening?" I think the question needs to be reframed to say, "should it be considered unethical?" because there isn't an objective thing that defines morality other than subjectivity which varies from person to person. But I personally wouldn't deem it unethical because by not allowing sentience to occur you're actually preventing the opportunity for suffering and perceptive misfortune from occurring.
When it comes to defining personhood this is solely a matter of semantics not really of defining morals. I would say what makes people unique is both memory and cognition. However, I don't know that I'd personally define a person by those things but to instead define a person as a particular individual conscious entity.
Gert, B., & Gert, J. (n.d.). The Definition of Morality (Stanford Encyclopedia of Philosophy/Fall 2017 Edition). Retrieved February 26, from https://plato.stanford.edu/archives/fall2017/entries/morality-definition/
Gloor, L., & Mannino, A. (2018, January 15). The Case for Suffering-Focused Ethics – Foundational Research Institute. Retrieved from https://foundational-research.org/the-case-for-suffering-focused-ethics/
Southan, R. (2018, July 1). Life: Why Bother? Retrieved from https://thenewinquiry.com/life-why-bother/
Your Brain on Tech - Mind Field S2 (Ep 4) [Video file]. (2017, December 6). Retrieved from https://youtu.be/1RHsAUyFCAM?list=PLZRRxQcaEjA7wmh3Z6EQuOK9fm1CqnJCI&t=864