Yes, in order to validate the chain of trust the TLD must be signed but sibling 2LDs have nothing to do with whether a particular 2LD can be signed. If wikipedi

Yes, in order to validate the chain of trust the TLD must be signed but sibling 2LDs have nothing to do with whether a particular 2LD can be signed. If wikipedia.org wants to sign, they can do so, because their parent (.org) is signed, but it doesn't matter whether widgets.org is signed.