It's impossible for us to provide an informed opinion. If the rent you paid was 2/3 of what the unit would rent to anyone else, it may be fair. If you, as a 1/3 owner and sister are paying below market rent, you may not want to stir things up or they could demand full market rent.
When a lease is not specifically renewed it goes to month to month, which is what you describe. When you are a 1/3 owner, you are not a 100% owner, so paying your siblings something isn't unreasonable. When you sell, the proceeds will likely be divided the same way. Should something break or need repairs, your sisters should each be 1/3 responsible for repairs.
Reconsider your daughters advice before you make any changes or hire an attorney. It would be a shame to ruin a good things and create hard feelings between your sisters and you.