She gets fed once a day about 3/4 a cup of dog food, and plays with her brother boston for an hour every day. He has no trouble staying fit and even gets fed more but she just keeps gaining weight!
You need to break that amount into two portions rather than one. Also walk her for 45 min in addition to play time with the brother. If no improvement then vet needs to rule out if she got some health issues causing the weight gain.