Two things that will happen next. Which happens first is not dependent on the other so I'll mention first the one with a definite time restraint.
A magistrate judge issued an order late last week for Tonya Couch to undergo a MHMR review:
http://dfw.cbslocal.com/2016/01/11/does-affluenza-mom-tonya-couch-have-mental-illness/
The CBS DFW report says that MHMR of Tarrant County has to provide their report to the judge within 30 days from when the order was issued.
In the event she is ruled unfit to stand trial at that time, the next part of that process generally involves treatment of some kind and a future reassessment of her fitness to stand trial. Treatment can involve medication therapy, in-patient treatment, etc.
The other "next" event is for the DA's office to present her case to a grand jury. A grand jury will either indict her on a charge or not. Most likely they will issue an indictment.
At that time the prosecutors could seek a re-arrest based on insufficient bond.
When the case will be presented to a grand jury is a guess. My guess is sooner rather than later.
After that and assuming she is deemed fit to stand trial, who can say how long before she goes to trial.
She does have highly experienced attorneys. Hard to say if they think delay is their best course or if they prefer a fast track to trial.
Can't say either way about which the prosecutors would prefer for this case.
As the defendant, she has the option of trial by a 12 person jury or a bench trial (trial by a judge only.)
But no, what happens (or does not happen) with Ethan's legal issues should have little, if any, bearing on her case.