Are family members a way to determine your size as an adult?

It's true that genetics does play a role in your future stature and health but it's only one of the many factors. Obviously it isn't the only determining factor or your mom wouldn't be a different size than her sisters. You will probably be small-medium framed. If you maintain an active lifestyle and eat a balanced amount of food- one that will feed you enough to keep your metabolism strong and healthy your whole life without over feeding yourself- and you are fortunate not to run into any thyroid or medical conditions along those lines, you probably won't have to deal with being overweight.