To answer to your current question;
it is very unlikely you'll find studies showing the differences between vaccinated and unvaccinated siblings, as the same kind of comparison can be made with people that don't necessarily share the same genetics: despite the immune response of an individual also depends on genetic factors, a lot of research need to be conducted yet. Thus, you'll indeed find countless papers reporting the data of vaccines' effectiveness.
I've also read your previous thread, and if her way of thinking is influenced by her family, then probably she lacks of basis knwoledge about how vaccines really work, the functions of the immune system (lymphocites, antibodies) and concepts like herd immunity. However difficult this may sound, you could try to give her a simple explanation about one of these notions; the majority of people reading a scientific article without having the simplest foundations of the argument being analysed, will always have doubts about its reliability. It's completely normal.
You can also find several information on the Internet that suggest you how to approach anti-vaxxers (there is a PDF published by the WHO as well) and then you can improve your own strategy for making her understanding.
