Complex sentence: Change.org operates a system for signature hiding, which only works if the user has an account on Change.org, however, it does not work if the signature was forged or appears on another site operated by Change.org, PetitionOnline.
Split into simpler sentences: Change.org operates a system for signature hiding, which only works if the user has an account on Change.org, however. It does not work if the signature was forged or appears on another site operated by Change.org, PetitionOnline.