There certainly are relationships between sign languages; A(merican)SL and French SL are arguably two dialects of the same language, or at least they were in re

There certainly are relationships between sign languages; A(merican)SL and French SL are arguably two dialects of the same language, or at least they were in relatively recent memory. Others are distantly related or unrelated; B(ritish)SL is absolutely unintelligible to signers of ASL and arose separately, for instance. Many of the modern global sign languages are related to one or the other of those families (and might be "dialects" as you want to call them, or might be further apart and thus merely "related languages"), but there are a few others that are not, including one—Nicaraguan Sign Language—that arose from scratch less than 40 years ago.