Yes it is. According to the manga, Giotto is Tsuna's ansestor and the others...it is also mentioned in anime too. There's a scene where Reborn shows Tsuna a family tree to prove how Tsuna is related to mafia in ep 1.
Reborn says that the Vongola Family's first boss, Giotto retired and crossed to Japan, so Giotto is basically Tsuna's great,great,great grandfather ^-^
So its not just a possibility, they ARE related.