Sometimes I have seen two young people start dating, and the interests of one partner gradually consumes all of the relationship.
Just because a guy is really interested in fixing cars does not mean that every date should take place in his garage working on one! Such courtships often end with the girl becoming no more important than one of the tools necessary for the young man's hobby or pastime. "Love (charity) seeketh not her own" (1 Cor. 13:5). And of course this kind of interest (or lack thereof) sends and unmistakable and painful message to the young lady.
While running an errand together to an auto parts store or spending an evening together working on a car might be a lot of fun for a couple once in a while, it will get pretty unappetizing for the sister if that's the only kind of dating that takes place. Herbs cannot develop and bear fruit where the weeds of selfishness are growing.
Don't take advantage of your girlfriend's desire to please you dear young brother. Make sure that your dating relationship includes activities that will feed and satisfy both.