To elaborate on what the others have said, as an interested person you may yourself asked the court to open a probate, if your stepmother refuses...

Am I able to revoke someone as power of attorney ?

You cannot revoke someone else's power of attorney. You grandmother will have to do that herself, and name you as the new agent.

Mineral rights & foreclosure; who's the owner?

This is sufficiently complex that you really will need to sit down with an attorney and have him or her review all the instruments and orders.

Do I need to inform any authority if my marital status changes after obtaining a J-1 visa, but before travelling to the US?

I'm going to switch this from international law over to immigration, where you will get better answers.

How can I take control of my mother's finances?

I agree with the other answers and that a guardianship/conservatorship is probably in order, although there is no guarantee that a court would...

This may depend on the nature of the seller. In Colorado a builder/seller is required to conduct and disclose a soils analysis. I would recommend...

If the heirs hire an attorney whose actions, and any ultimate result, are to the benefit of the estate and other heirs as a group, they definitely...

How do I legally decide who to pay first, when I don't have enough money (yet) to pay all of the contractors I have?

I agree with Robert,. But just to make clear, it is not a question of "legality" but strategy - although if you were to pay some creditors and not...

Can I sue my uncle. ?

I agree with the others that more facts are needed. But if this was an arms length transaction and you and your sisters were at no disadvantage...

When I sell a signed photographic print through an art gallery, can I bypass the gallery and sell identical copies of the print?

If I am understanding your question properly, I think that will depend entirely upon the agreement or contract you that you have with the gallery.