There seems to be several layers of this situation that you will want to explore. First, your feelings are not random. How is it that she is getting proposals left and right? This doesn’t happen out of the blue, so your concern is real. Men typically don’t propose marriage to someone they have no relationship with — so you having concerns makes sense. It also sounds like she is from a culture where arranged marriages are organized by the family. This concern of hers that you need to ask her brother now brings the question to you if you are ready to get married. Are you? This may be something she and her family want, but is it what you want at this time?
Finally, I would have a very open discussion about your concerns and insecurities. This is in the service of both of you ironing out your concerns, needs and fears. From there you can make a decision about moving forward.