My introduction... and problem

Hi everyone,
my name is Ted and my wife and I own a dachshund mix and a mini dachshund. The mix is 6 years old, and she is a very smart dog as well as avid hunter of all critters large and small. Her name is KC and she just a wonderful dog. Our mini is 7 months old and is a beautiful dog. She loves having an older sister and is a beauty queen of a pup, but Sophia is struggling to catch on with the potty training. I have her on a feeding schedule. She is in her crate while we are at work, but we try to keep her out of the crate as much as possible when we are home at night and this is where the struggle begins. She knows to go potty outside before and after work, but during the evening I have to literally watch her at all times to prevent an accident. Can anyone give me some tips on helping her to get it? KC was easy to train, so we don't know what we're doing wrong with Sophia.