Some A/B trusts are very hard to read and administer.
I would hire an attorney to form an opinion of the correct transfer
of assets upon the first death and write a letter
to your Dad if it wasn't done correctly and give him a chance to correct.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
If under the trust the funding of the B trust was mandatory, then yes, not funding it would be considered a breach of fiduciary duty unless everyone interested in the trust agreed otherwise (which could have some tax implications). Your father can, of course, start a new trust, but he (or his estate) would be liable for not placing mother's interest in the prior trust into the B trust.
It is, of course, extremely difficult to bring an action against one's parent, which would be the course of action if your father does not respond well to a request for him to go back and make this right.
Do you think that your brother is improperly influencing your father to gift him property? Does your father lack cognitive capacity? Is he easily influenced? If so, this could be a matter for an elder law attorney.
As these comments often do, I'll end by saying that a consultation with an experienced trust an estate attorney is merited. How to respond to these types of issues requires not only careful legal analysis, but also a discussion of the impact on your relationship with your father.
I think you should take the Trust to an attorney. If the terms where that Mom's share were to go into a Bypass trust, then they should have as other counsel have said here. An attorney will have to review it though to make sure it was mandatory and no by disclaimer or voluntary. Also, the trust may have been amended without your knowing it.
At a minimum, I think it is worth your while to sit down with an attorney and review the Trust language and what Mom's estate consisted of. Good luck.