That's just the article's handwaving. The case report actually says:
> During follow-up at 3 years of age, routine ovarian surveillance showed Twin 2 to have gonadal dysgenesis, and prophylactic oophorectomy was performed.
Yea, this is highly clarifying. The OP article makes it sound like they decided to render Twin 2 sterile because of a risk of cancer inferred from some theorizing about the genetics of a these new sorts of twins. In fact, it sounds like Twin 2 was already sterile.
> Streak gonads are the defective development of the gonads in an embryo, with reproductive tissue replaced with functionless, fibrous tissue, termed streak gonads. [It is] a form of aplasia, resulting in hormonal failure that manifests as sexual infantism and infertility, with no initiation of puberty and secondary sex characteristics.
> During follow-up at 3 years of age, routine ovarian surveillance showed Twin 2 to have gonadal dysgenesis, and prophylactic oophorectomy was performed.