If my father passes away before my stepmother, can she amend the trust and remove one or both of us as beneficiaries?

I cannot tell you what the trust provides without reading it, but it is common in a remarriage situation to use a trust to ensure that, regardless of the order of deaths, after the second one dies the property descends to the persons they intended. This often involves something called a "QTIP trust" coupled with a provision that makes part of the trust irrevocable (not subject to amendment) after the death of the first spouse to die. In the typical second marriage situation, part of the property ultimately passes to the children of one spouse and part to the children of the other spouse, but the variations and fractional divisions are infinite. It is only after the second spouse dies that anyone else receives anything, but there are exceptions to that, too.

You have to look at what the trust documents state. Since it is revocable, there is a good possibility that upon his death, she can revoke the trust and do what ever she wants with the assets.