Before you speak to an attorney make sure you have copies of everything. All the trust documents and all of the portfolio. Then read those documents. More than once might be a good idea. Make notes of anything that is not crystal clear to you. That way, when you do go in to speak to the attorney you will know exactly what your questions are.
And no, if something happens to your brother his widow cannot somehow cheat you out of your share of the estate. Well, she could if you stand by and don't do anything. But that won't happen, will it? Once concern you might have would be that it could be possible that everything will be spent by the time your mother dies.
Do make sure you know exactly how much is currently in the trusts and just how much of it is being spent each month for your mother's care. Which means you'll have a rough idea of what would be leftover down the line.
You are not being greedy. You are simply wanting to establish your place in the inheritance. And the more you know, the better off you are.
Do not sit by passively on this, but do what you need to do to clarify everything.
A personal note. A couple of years ago I re-did my own will and trust work. I sent copies of everything to my son, right now my primary heir, and also to my sister, because she will get part of a life insurance policy, or everything if my son were to pre-decease me. I also sent my son a copy of a recent month's copy of my investments. I honestly feel that being completely open about everything made the most sense.
It does feel as if your brother is being pretty open to you, and I hope everything works out well in the end.