3rd DWI. What can we expect?

Have you seen the citations that she is charged with? Normally if there is a child in the car the driver is also charged with endangering the life of a minor.

In my experience, most prosecutors will not agree to something that does not involve jail time and a felony conviction. Rarely is there an offer which does not include at a minimum completion of SAFP (also called Safe-P or SAFPF) SAFP is a Substance Abuse Felony Punishment Facility. It is a lockdown, mandatory treatment program. I have seen a lot of success with the program. But I also realize there is no "one size fits all" program that works for everybody, and the success is also determined by the participant's attitude. The program generally lasts about 6 months. After the SAFP most of the time there is three months of half-way house re-integration and then a drug court aftercare program.
Its not easy. Overcoming any addition is not easy. But it may be the best thing that ever happens to her.
Good Luck

Your sister is WAY over confident. She is in the land of those who love to prosecute DWI cases. She could be filed on as DWI with a child but it is not a worse charge than felony DWI. But, the fact she had a child in the car will NOT sit well with the prosecutors. There is a good chance that IF she gets probation, she will find herself in a strict treatment program. Moreover, she WILL lose her driver's license. Probation on a felony DWI is nothing like probation on a misdemeanor. It will not be easy.

Your sister is in for a rude awakening. She needs to communicate with a criminal defense attorney in Montgomery County ASAP. In other parts of Texas, someone facing a DWI 3rd is regularly offered prison time. They are not "just going to get probation." The facts that she had an accident, had a minor in the car AND the child was not properly restrained, not only strengthen the prosecutors case, but also angers the prosecutor and potential jurors. I regularly see people offered 3 years to do on a DWI 3rd. With the "aggravating factors" on your sister's case I would not be surprised by a stiffer offer. BUT there may be silver lining that only an experienced criminal defense attorney from this jurisdiction would potentially discover.

The offer from the State will hinge largely on the evidence that was gathered in the course of the investigation and following her arrest. Presumably, the police obtained a sample of your sister's blood for testing (as is the case in most felony DWI investigations). However, if the officer did not first obtain a warrant for her blood and she did not consent to the blood draw, there is a good suppression issue that may keep that evidence out. If the blood evidence is suppressed, she looks decent on the video (if there is one) and she didn't make any damning admissions (i.e. "I took 3 Xanax and 2 Soma, what's the big deal?") then there is a possibility that the felony DWI charge could be reduced, dismissed, or amended to a different charge which could lead to a probation offer.
That said, it is imperative that you retain an experienced DWI attorney that practices routinely in Montgomery County IF you want to give her a fighting chance. When interviewing attorneys for your sister, be sure to ask if they've ever handled a McNeely motion. If they've never heard of it, find another attorney.
It sounds like your sister needs professional help for her alcohol/chemical dependency. I pray for your family that she does get the help she needs regardless of the outcome of this case.