Sistergirl and Brotherboy are terms used in Australia to refer to Aboriginal and Torres Strait Islander peoples who are gender diverse. The terms are not analogous to transfem and transmasc as understood in non-Indigenous LGBTA community, and were coined directly by Indigenous people.
A person who describes themself as a sistergirl or brotherboy is understood to live their life through gendered experience that isn't consistent with their birth sex. This includes gender identity but also cultural identity, social role, and other gendered aspects of the Indigenous cultural worldview. Because of this, the terms do not explicitly refer to people who may otherwise be described as transgender; a person who is a sistergirl may be a trans woman, a transfem nonbinary person, a feminine gay man, a drag queen or any other permutation. Sistergirls and brotherboys may not consider themselves to be identified as the gender the word may imply; one may be a sistergirl but not a woman.
Like many concepts of gender from indigenous peoples, the Indigenous Australian concepts of gender, including sistergirls and brotherboys, is not best understood in terms of western gender-descriptive language, and should not be misunderstood as being a term for transgender people. Indigenous culture surrounding gender in so-called Australia was affected by white invasion which attempted to enforce gender role based on physicality rather than spirituality - based on body parts instead of internal gender experience. Indigenous culture around Australia varies and many sistergirls and brotherboys are subject to homo- and transphobia in their communities, a significant deal of which is due to colonial enforcement of strict gender.
"Indigenous" with a capital "I" is the correct way of referring to Indigenous Australians, as opposed to indigenous with no capitalisation, which refers to native peoples in general.