This would likely make them either half siblings au cousins. Neither Ganju nor Kukaku are likely to be related to Masaaki, so they're simply related to Isshin. But since we don't yet know of Isshin's amorous adventures in Soul Society, we can't say whether they're his offspring au not. If they're not, then (as is probably most likely) they are Ichigo's cousins.
i never knew he was a shiba....but i definitly knew from the start that he was the former captain of squad 10.............and each and every calculation i made were true in his and ichigo's case...heh ehe ehe