It’s an unnamed person/another way of saying somebody if you don’t want to mention names. There are 2 possible meanings I can think of depending on context in this example: Meaning 1 - I would assume that the so and so refers to someone well known who has a brother (and that brother is always known as the brother of that person (the brother of “so and so”) - rather than being well known. e.g. “Whilst growing up children of celebrities get tired of only being known as so and so’s son/daughter.” Meaning 2 - it can be used to refer to name of someone who has been forgotten or ignored (or implying their name is not worth mentioning). So he’s so and so’s brother could imply that you’ve forgotten his brother’s name or that you consider his brother’s name unimportant. E.g.: “they just got some so and so from the head office to come and give a presentation”. There are other meanings of the word “so and so” that wouldn’t apply to your example but may be useful to know. E.g. a type of person, usually implied a bit insultingly - e.g: “he’s a really nasty so and so” (it’s a polite way of saying you’d like to use a stronger insult but replace with “so and so” instead!)

"So and so" is where the name of the older brother or sister would go. It is a filler word that is used when referring to a person in a hypothetical situation. So instead if people calling you by your name they say "omg!!! You are Ben's little brother. Hi!! Nice to meet you!" Your sense of identity is lost because people see you as your older brother or sister's younger sibling instead of by your own name and identity. Also "so and so" also replaces the name of another person's name especialky if you forgot the name. Example: A: hey i heard you were sick!! B: really? Who said that? I was just traveling. A: oh um..so and so said that....hm..i forgot their name.

So and so just means 'the name of their elder brother or sister' Jack will always be known as David's younger brother. Mary will always be known as Sarah's younger sister.