If a minor is pregnant, can they legally live with the baby’s father and his parents?

There is no absolute prohibition against cohabitation under these facts.  However, the result could be different depending on the differences of the age between the minor and the father and any pending criminal charges. 

Texas has statutory rape laws, but also have "Romeo and Julliet" defenses.  This means that if the minor is over 14 years old, the two are within three or five years of age (depending on the charge), and the sexual acts were consensual, then the defendant would have an absolute (or affirmative) defense to the charges, ...then no one would complain about the parents of the minor trying to help the two kids try to work through the new world of parenthood.

On the flip side, if criminal charges were pending, and orders had been put in place that required to the father to stay away from the minor-- then the cohabitation would be illegal.  The order could be in the form of a protective order, a bond condition, or a temporary order.  Many people ignore these orders, not realizing that even a victim can technically be charged with violating a protective order that has been entered for their protection.

If the case does not involve teenager range kids, but rather a minor with a much older male (like a 40 year old man), then the situation would be much different.  Because there is no defense in this situation, the parents could be charged for failing to report sexual abuse or as a party to sexual abuse if they continue to allow a the father access to the minor.  At the very least, CPS could get involved and remove the child for a continuation of child endangerment.