So are all the Sohma's related?

There are Sohmas who are related por blood, others por marriage. Like Kana... she is a sohma, but not part of the curse. I think that the bloodline is very big- and so there are many of the "sohmas". If you are talking about the zodiac< i think they are indirectly related... different families, but some of the same blood ( i like to put it as they are totally different from one another but just with the same last name). So yes... i think they are all somehow related, even if it's not genetically.