You should be checking Greyhound's website for policies related to travel. For example depending how old your sister is she may be falling under the Children Travelling policy. Normally if she is a resident of Texas and over 15 years old she can have a driver's license, which is an ID if not according to policy she has to be accompanied by you and it would be fine. The only thing that may be required is that she has a letter from your parents that she is allowed to travel with you, while this is a definite requirement for out of country travel without both parents this may be a requirement from your state (see Illinois exception for Greyhound's Children policy).
As far as the baggage is concerned if I understand it correctly you have a total of 3 pieces which should cost you only $10 according to policy.
As far as general attitude of people at the stations depends on the person you encounter and if someone want to give you a hard time you will certainly get it.
As far as police stops are concerned. Question is you which direction your bus is going to even be an issue but again. If she has your parents permission letter there shouldn't be any issue.
EDIT
Express Reservation as far as I can tell is reservation for Greyhound Express with all the amenities.