As a younger sibling myself surrounded by other younger siblings (my Dad was the youngest by 12 years and my Mom is the youngest of 3), I know a little bit about it. I know that it’s possible to be raised a little differently than your older sibling(s) and these characters can be written in a specific way. Most of the time, the younger sibling is written as “annoying” in order to show how hard the protagonist has it at home.
The younger sibling is not always “spoiled” or “annoying”. It’s true that parents might be a little bit more relaxed with rules by the time they get to their second or third child because they’re been through it all before. They might not be as restrictive. First time parents might often be nervous and watch their babies closely at all times in fear of them getting hurt, but they seem to become more relaxed with children they have after that.
Authors sometimes write younger brothers and sisters as tagalong characters or ones that are constantly causing trouble for the protagonist. The dynamics between siblings are never that cut and dry and there are ways to make it all interesting. Sure, older siblings might become jealous of the attention that their young siblings get, but that doesn’t ALWAYS happen and most of the time it goes away.
Be willing to experiment with relationships between characters and don’t always go for the clichés. If you have a younger sibling that was particularly annoying as a child, you can use those incidents to make your story more realistic. It’s perfectly fine for your siblings to get along fine and work together in your novel. Relationships become more interesting when you switch them up.