Cedarhurst, Yes, if you don’t tell the people behind you that you are holding the space for your mom who is coming with a load of groceries it is wrong. IF you do tell then it is the same as standing there with the loaded wagon, and the other person can choose whether to wait behind you or go on another line. Obviously if your mom does not come back when it is your turn, people behind you will go first but you will have to continue to say that you are holding the space for your own wagon full of groceries.
In that way you are not being gezeilus daas. You are not fooling or cheating anyone.