Rosalind Nott's purpose in Rodolphus" horrible plan became quite clear. Thanks to Rodolphus" inability to father children, he had to rely on his younger brother to provide him with an innocent child to sacrifice. Offering up one of the children he shared with Gemma would've never been an option. Rabastan loved his own children too much to offer and the only one who would likely be innocent enough would be his youngest Gwendoline. It was obvious to everyone that if he had a favorite, she would've been it. No, Rabastan wouldn't even do that for his brother.