The short answer is yes, Ishara was Tasha's Sister.
The evidence, both direct and circumstantial is pretty overwhelming;
The script and screenplay list her as being Tasha's sister, the (isomorphic) DNA comparison run by Crusher indicated that they shared familial DNA and the DNA alteration technique mentioned in Bloodlines would have left her with noticable neurological damage, none of which showed up in her scans.
Picard : Doctor Crusher just called me with the results of her DNA comparison. Ishara is Tasha's sister.
It's pretty unrealistic to imagine that the rebels would have sufficient access to the crew roster for a Federation vessel to identify a crewmember who'd recently died, somehow locate her DNA and then implant that DNA into a fake 'relative' that (hopefully) that crewmember had never discussed with her colleagues. Also Ishara's stories of her and Tasha's early life in the colony tally perfectly with what the crew had been told.
