It’s possible that, once you combine all these words, it gets easier to see a connection. But are they really connected, or is the similarity just a coincidence?
You very well know the meaning of the word «sir», and probably know that «ser» is an alternative spelling used quite often in fiction, fantasy, etc. (of course, Game of Thrones and all that kind of stuff come to mind).
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Now, the word «senior» is also used in English to refer to elders in a respectful way. This word undoubtedly comes from the Latin senior, which is the comparative form of senex ‘old’. This is why it is also used to oppose Junior and Senior when talking of a son and a father with the same name: it literally means John the Younger and John the Older.
The Spanish word señor ‘mister, master’ also comes from senior, although it is not connected with age, but with respect (after all, that semantic transition makes a lot of sense).
So going back to «sir» and «ser», do they have their origin in the Latin senior? Well, yes, they do, specifically through the fuller word «sire», through Old French sire, another form of sieur, which you can connect to monsieur (literally my sieur). This sieur is the French evolution from Latin senior.
