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.
There is good Yiddish word for this, mishbucha, which according to my Jewish friends, means someone who is related to you, usually in a complicated way.
Best Answer
There's the term "prospective step-siblings", which implies a significant possibility that the parents will get married. You could also precede "step-siblings" with a variety of modifiers, such as "quasi", "pseudo", "para", "almost", etc. You could also apply modifiers directly to "siblings", either the above, or another such as "meta", "allo", etc.