Neither is the most "correct" take on this, adding characters that are related to your original character can both add to a story as well as take away from it.
There's nothing that inherently does either (except in the most objective sense of what it means to "add" to a story), but how you execute it is entirely what decides whether something is "adding" to your story or "taking away" from it. If you've said your character is an only child and you're afraid that introducing a sibling that you feel would work really well as a way to continue that character's story would contradict what you've built into that character's backstory then you can utilize the freedom of being the
author of your writing and make your character wrong - either by having a sibling they didn't know about, or having had their parents remarry, or even entirely circumvent the more conventional idea of what makes a sibling and introduce a character that is so "sibling-like" that the narrative goal of having a sibling is still fulfilled with them. Doing it poorly would be, as you probably imagined, anything that doesn't make any sense with your character's history in mind.
Pulling it off properly is what makes it "add" to your story. The only time it takes away from your stories is when you introduce it, or execute it, in such a way that it feels cheap or poorly done - but that's something rather subjective that I think matters more to the writer than to the other people they're writing with. The people you write with, the people who read your writing, don't have an encyclopedic knowledge of your characters' lives and won't know if you fudged up and willed a new character or relationship into existence "badly" unless you've explained that to them. Even those who think you maybe pulled it off badly or at least not as well as you could have probably are still operating off of that limited perspective and might not have information that could change their mind on that, too.
All in all, it is better to do what is best for you, if that means writing "legacy" characters (a sister, a brother, a child or grandchild) then you do you and just forget about worrying how people will see them. What matters at the end of the day is whether or not you enjoy writing the posts you make and if you think that introducing a new character or many of them will do that for you then it is worth exploring, and if it doesn't then don't.
Kitter Bitters