Your question is too vague about interested parties to be able to answer. Anyone who would be either an heir at law or a named beneficiary can...

If she has a guardian appointed over her person, she cannot legally consent to marriage.

You need to have an attorney involved and do more due diligence into the legitimacy of this person.

You need to file a suit to have the house sold and the money divided.

You need to see about hiring an attorney to have the guardianship set aside.

You you were the pay on death for his bank account, and the named beneficiary for his insurance, then you wouldn't even need to worry about...

You can be listed as a lien holder on the title.

I think you would need to file for a guardianship and give notice to the biological father to do that.

The grandmother needs to legally adopt them.

If she hasn't opened a probate you can open one yourself.