Nurse here, that has worked many years geriatric.
Buy the water proof mattress pads that are about 30-36"" long and a little wider then the top of the mattress you are using.
That way you can tuck them under a little.
Next take a few older sheets and cut them so they are just longer then the water proof pad but keep it so that you have plenty to tuck in the sides, that covers your water proof pad and keeps it from moving. Some places even use quilted pads between the rubber pad and that top sheet you cut (draw) sheet. More comfortable in warm weather then sleeping on something that is rubberised.
Double check before restricting fluids as many elderly get dehydrated easily. There is also medications that help with control. Also check the meds she is on as some diuretic's may
be causing the problem and hours she takes her meds can be changed.