When a child is first learning a language in a bilingual environment, is it easier or harder to properly acquire the two distinct languages if they are more similar? For example, is it easier for a child to learn Italian and Spanish growing up in an Italian-Spanish household or English and Japanese growing up in English-Japanese household?
Four cases are possible:
[Easier] It might be that similar languages require similar vowels, grammar, and/or vocabulary (or many other factors, the specifics don't matter) and thus are easier to learn.
[Harder] if the languages are close, it might be harder for the child to learn to distinguish them and thus they might develop a mixed-language instead of learning the two separate languages.
[Same] It could be that children's early language adoption is so robust that there is no statistical difference between the two categories.
[Mid] There might be a sweet-spot where the language are distinct enough to not be confusing to the child, but similar enough to allow recycling of learning. If you pick more similar or less similar languages than this sweep-spot then the acquisition deteriorates.
If the case of children is different from adults (i.e. if similar languages are not easier for them) then why is this the case?