How long ago was it when you were small? That may help in getting a timeframe for the movie.
The first thing that came to mind from your description, though with noticeable differences, is The Little Dragons (1979), put out on DVD as Karate Kids USA.
The main differences from your description is that the girl kidnapped is not their sister, but a love interest of the older brother he meets when they go to an RV park with their grandfather (I don't recall if they were sent off with him by their parents or not, but they are not with their parents through the movie). They do wear standard white karate gis, though pics I have seen for a DVD cover or movie poster shows the elder brother wearing a red belt. The guys doing the kidnapping are hillbillies who, I believe, hold her captive in their house; and their overbearing mother is played by Ann Sothern.
There is a motorcycle gang is this, but the kids don't fight them, they actually help the kids.
I get the impression that kidnapping a girl and having the karate dudes come to the rescue is a common theme is these types of movies.The Little Dragons