In the Next Generation episode Legacy the Enterprise crew is introduced to someone that they are told is Tasha Yar's sister, Ishara Yar. The crew is cautious at first, but after a DNA comparison by Doctor Crusher confirms Tasha and Ishara were sisters, everyone readily accepts Ishara.
But was Ishara actually Tasha's sister? We find out later, in the season 7 episode Bloodlines that it is possible to alter someone's DNA. And considering Ishara's and the Coalition's objectives and modi operandi, it would not seem beyond their morals to perform such a procedure on someone just to gain an edge over their enemies.
While memory-alpha doesn't list there being any doubt in Ishara's relation, this is a question that has always bugged me about this episode and character.