Your feelings are your feelings. I can understand the desire to protect your brother. Consider different options that explain things rather than the “taking advantage of” scenario/motive your fears are driving you to think about this girl and thus limiting your options.
My advice would be that your brother is an adult and adulthood is being responsible for one’s choices (and respecting others’ choices). Let him make his. Pray for him and do your best to adapt to his choices. Talk to him man to man about your fears if needed, but don’t blame or get accusatory.
E + R = O
the event is the girlfriend and brother relationship
your response is currently being fearful and suspicious
the outcome is that you consciously and subconsciously create drama and division driving a wedge between you and your brother over this
The only thing that you can’t change is the event. Your response can be any number of different (and healthier) things like simply asking your parents and brother for family time, getting to know this girl since she will be around, bringing your girlfriend around more so you’re not the only one at dinner without a significant other, creating other quality time with you and your brother, and so forth.
Change your response to the event and you will alter your outcome. You and everyone else will be happier. Trying to change the event (your brothers relationship) will end in disaster.