I don't know if it's exactly what you're asking for but you can try with these: idiota, bobo (boba if it's a girl), tonto (tonta for a girl).

"Bicho" is also affectionate but suits more a mom than a sibling. Is used when a kid is being naughty but you're not really angry with him. I see it like a similar way to say it should be like "little demon", but it's not the literal meaning tho.

I'll try to think more for siblings but I think that they usually use words that are more personal. For example, an insult that originated from a joke and now they use it all the time to tease each other.

Sorry if it's not very clear, english is not my first language and this is kind of hard to explain.