Her allowance is paid out of the income from the Windsor's family Land
We don't have a dismissable monarchy, it would require great constitutional change to get rid of the Royal's, and even then they would still be the monarch of 15 other countries and head of the Commonwealth
Just how did they come by this land? Was it the land that belong to the monarch that they inherited when George I was offered the crown and the House of Hanover took over from the Stuarts as monarch? In that case it's not really their land, but rather the land of the monarch, whoever that may be. Its wrong to think of it as the Windsor's family land in that sense, rather the income from that land is used to fund them and their endeavours. If the monarchy were to be removed I would see those lands as largely reverting to the state.
How the current owners of land came by it does not bear scrutiny _anywhere_. See: Conquistadores, American wars against the Indians, etc. The best we can do is accept the current state of play and remedy the worst inequities to a degree.