How are relative familial titles used for members who died prior to your birth?

I would ask whether you would have any issue referring to, say, your mother's grandfather as your great-grandfather, even though he may have died some years before you were born. Your mother's brother is certainly your uncle, and in my opinion it's correct to refer to him as such even if you never met him.

Your reluctance to use the word "uncle" in this case may be because it is common to think of our uncle (or aunt) as a relative perhaps second only to our parents in closeness and affection. Consider the word "avuncular" which means literally "pertaining to an uncle" but has a secondary meaning of "kind, genial, benevolent". Having never met your mother's brother Bruce you might not feel comfortable referring to him as Uncle for this reason.

The issue you raise of an uncle who died in infancy is an interesting one. Again it's probably because we typically know our uncles as men of our father's age that this seems odd. If you're a "Seinfeld" fan you may remember a scene in which George is talking to his mother, who mentions "Aunt Baby", a relative who died as an infant. This has always struck me as humorous, yet quite poignant.

I would say use the title "Uncle" freely and with the respect due to a close member of your family. My mother's brother died in a Japanese POW camp in World War II, some 25 years before I was born, yet I proudly refer to him as my Uncle Arnold.

How about referring him as:

Late Uncle Bruce

If your father died before you were born you would still call him your father. Same for your brother or sister if they died before you were born. If they died before you were born you would never say: My mother's son / daughter. Therefore, to me the title remains even if you were not born when they died.