How come I have sarcoma, but nobody else in the family has it?

Rare. Sarcoma like many cancers do not have familial predilections. They are sporadic and rare which is good.
Unknown causes. Most cases happen without a clear cause; some cases happen in patients with familial cancer syndromes such as li fraumeni syndrome but in the large majority most cases are not inherited in families- as such this situation is a sporadic case.