Can POA refuse a surviving spouse's claim to inheritance?

Your husband's sister's power of attorney can give her only the authority to act for your husband while he is alive. All powers of attorney, including "durable" ones, which remain effective if the principal (here, your husband) becomes incompetent, must and do end once the principal dies. So the sister's POA cannot be used to prevent you, as surviving spouse, from an inheritance you would otherwise be entitled to.

You say that the house and the motorcycle are in both your names. Hopefully, this means that you hold title "in joint tenancy"or "as joint tenants." [Or perhaps as "tenants by the entirety."] In that case, your husband's share of the property passes automatically to you, outside of probate, when he dies. If you hold the property in some other form of shared ownership (generally called "tenants in common") your husband might have the right to leave his interest in that property to someone else. This would depend on the marital-inheritance laws of the state you live in. Of course, if he left you his share of the property, by his will or otherwise, there'd be no problem here.