I'm having a hard time deciding if I have a valid traffic stop and hoping ya'll can offer some help.

Scenario: At 1:00 a.m. officers investigate a criminal trespass. The victim reports the trespass and identifies the suspect by name. Suspect is nowhere to be found. At 3:00 a.m. victim's sister calls PD reporting that the suspect is within her eyesight, again trespassing on same property. While on the phone to dispatch, sister identifies herself, describes what the suspect is wearing, and states that the suspect is now leaving the premises in a "brown 4 door" car heading north down certain street. Within about 5 seconds (we're obviously in a small town) officers spot a "brown 4 door" and stop the car to investigate the criminal trespass.

Turns out that the driver of the car (sister of the trespassor) is drunk and this becomes a DWI arrest. Perhaps I'm wrong, but it seems that we can overcome the problems associated with anonymous tips because the caller is Id'ed and offers a decent description of the car (maybe). I'm more concerned about the validity of stopping the car just to investigate an out of officer view offense.

I think you're fine. Police just need reasonable suspicion to effect a traffic stop/investigative detention, and a vehicle matching a description given in the caller's geographic area would rise to that level (at least, in our courts it would). Now, arresting for the criminal trespass without a warrant at the time of the stop would be problematic, IMHO.

Look at a FW case called state v sailo aka mailo and check the cases citing it.