Halakhah for Sanhedrin 127:3

From the laws of the commandment is that which they, may their memory be blessed, said (Sanhedrin 64a), "[If] he gave over but did not pass, passed but did not give over, he is exempt"; that he is not liable until he passes part of his seed and leaves some of it, as we said; that he is liable whether with his fit seed, or whether with his disqualified seed; and that he is liable for all of those that 'come out of his thigh' - sons or daughters, whether children or grandchildren, forever. But if he passes his brothers or sisters or fathers or passed himself, he is exempt. And the rest of its details are in the seventh chapter of Sanhedrin (see Tur, Even HaEzer 86).