If they split their time b/w you and their mother, it means that they do not live at one house exclusively. That makes it harder for you to have your privacy even though the two girls are quite older now. Unless there is a definite living arrangement either at your house or at their mothers or else where, they would have to be able to come to you.
It seems that you're referring more to their boundaries of not following your rules and taking advantage of the situation more so than you being upset of the go in between arrangement w/ their mother. The girls have to respect you and follow your expectations. They are still dependent on you and your mother. Even if one is 18 yol, if she does not have her own living arrangements elsewhere (college campus), she is to respect your boundaries. Perhaps you would have to present that to both girls differently. Speak to them about responsibility and come up with consequences if they fail to do what's expected. Living the house unlocked is unacceptable and ought to carry a consequence.