I'm going to edit the practice area of your post to "Criminal Defense" for better exposure.
This attorney is licensed to practice law in the State of California. The laws of your jurisdiction may differ and thus this answer is for informational and educational purposes only and is not to be considered as legal advice. Since all facts are not addressed in the question, this answer could change depending on other significant and important facts. This answer in no way constitutes an attorney-client relationship.
The laws concerning this case are very specific to your jurisdiction. My concern is the "mostly" part. The threshold for a criminal investigation is very low particularly when there is a child involved. That said, the offering of weed may be a crime as well. Your brother should be consulting a criminal defense attorney if he believes that there is any possible investigation concerning those emails and text. In New York, his actions would certainly be investigated if a parent made a complaint.
No, no one can be sent to prison for life because of this. But saying he could get her weed is a huge problem for him. It does not matter than he never furnished the marijuana; his text show the beginnings of negotiating a sale/furnishing of a controlled substance to a minor. Plus, it shows that he has access to illegal drugs (since he is not doing this in the context of medical cannabis).
Remember, too, that 13 year olds could easily say that he did furnish the m.j. if they are seriously pressured by a parent or think it will get them out of "trouble" in some way. The allegation alone could put your brother on law enforcement's radar for a long unpleasant ride.
The optimal approach is to hire an attorney asap. The attorney can conduct an investigation into this matter before the police conduct an investigation. What your brother needs to stop doing is talking to anyone but an attorney about this matter. You also need to cease talking about this. Your brother obviously needs to cease any communication with his wife's niece (or her mom). As far as whether this is a chargeable offense, more information is needed (to not provide it on this public forum). Get an attorney experienced in conducting pre-filing investigations. Best of luck.
The first major question is what is a 31 year old doing texting to a 13 ?! (Don't answer that). If there is no express or implied sexual connotation to the texts, then he likely does not have to worry about any criminal charges of a sexual nature. However, the references to marijuana opens up a pandora's box of potential charges, not the least of which can be "endangering the welfare of a minor". He should stop any communication, of whatsoever type, nature and/or manner conceivable, both with the minor and the parent, and should consult with t a criminal defense attorney. H
He should have an attorney (and maybe some bail money) lined up in case the police come for him. At a minimum, he should not resist arrest, not consent to any warrantless search of his person or property and he should not talk to them at all other than to politely request an attorney.