Board Thread:Questions and Answers/@comment-36015352-20190127222059/@comment-995426-20190128003224

Ms.HamatoAlexander is right&mdash;it's something worth considering, but never really addressed in depth. If true, though, it would also make 2012 Leonardo and Miwa blood part-siblings, and banjo strumming happens.

The Turtles (Mirage) and the Turtles (IDW) may be blood brothers as well, irrespective of their mutation source.

But for the Mirage Turtles, blood relation doesn't matter, as their father is not blood-related to them in any way and is still their father, and the four sons are brothers to each other. Gradually the family expands, as becomes a big sister figure to the turtles and a daughter-like figure to Splinter, and Splinter eventually fully accepts her as his daughter at the end of the original City at War arc in the early 1990s. And for a time went on his own out to Colorado and met and married Gabrielle, who was already pregnant from a previous relationship with an unknown man. When was born, Casey was automatically her legal father. But Gabrielle died in childbirth, and Casey took Shadow with him back to New York, where he found April a second time. Casey and April had never successfully started a romantic relationship before (one of the reasons Casey left for Colorado), but this time they not only got together, but got married, and April legally adopted Shadow. And all eight characters&mdash;Splinter, Leonardo, Raphael, Donatello, Michelangelo, April, Casey and Shadow&mdash;are a three-generation family; most are not blood related to each other, but it doesn't matter, because it's all a beautiful story about adoption creating families that are just as real.

The IDW Turtles' blood relation is also specifically irrelevant, but for a different reason: They are all reincarnations of the previously human Hamato Sons reborn as laboratory terrapins, and  is the reincarnation of their previously human father  reborn as a laboratory rat. Splinter recalled his past life in full even before he mutated because the psychotropic compound turned him into a sentient lab animal while also heightening his spiritual awareness, but their subsequent accidental mutation also put the four turtles on a course to gradually remembering their own past lives and remembering what the family members mean to each other. So they were blood relations in their past lives, and that is the only relation that matters.

I seem to recall that 1987 TV series specifically ducked all questions of family between and the, but because of standards and practices for children's television at the time:  Splinter would have essentially been a single father of a different race raising four teenage sons alone, and a majority of the American public at the time considered those kid-unfriendly ideas; they by and large still did not morally accept single fathers or interracial families. As a result, those Turtles were essentially treated like college roommates, and Splinter was their teacher and their dorm keeper. Splinter would sometimes address each of the Turtles as "my son," but he did this to practically every character of sufficiently young age, making it more a gesture of elderly kindness than a gesture of family. For a franchise built so strongly around concepts of family, that series bent over backwards to erase any semblance of the main characters' family structure.

But I may have rambled on for too long. You probably only asked the question through the lens of a single television series, and I made a franchise-wide in-depth analysis.