It will be revealed in this ark that Miyuki´s body was genetically engineered to be perfect, so even having kids with her brother represents no risks for the kids.
Also, They ARE blood related brother and sister, but by the end of this arc they will be engaged, and Maya will make it public a lie that Tatsuya is her son (making Miyuki and Tatsuya supposedly cousins), but that's just a lie to make the engagement possible. During the 1on1 talk of Tatsuya and Maya she will say why she considers him his son, but don't want to spoil it