I would talk about you were able to observe dentistry when you were young, in respect that you actually did. Talk about what did you like, what did you observe, and who you were specifically observing. You don't have to mention that he or she is decreased, but if you do it really sets a motivation to pursue dentistry because you want to continue the legacy that the person left. That is only if you are being honest, that's how I will feel if I have a decreased person doing dentistry. You also don't have to mention that the person passed away.
Overall, it is about keeping it real. Admission committee will see through it if you are not. If you are not comfortable talking about your deceased relative, then I suggest not go through this path.
