From experience, I know that sometimes it can be a challenge to get some older people to drink liquids. First, ask yourself how much liquids your mother should be drinking.
Most adults need around 6 to 8 eight ounce glasses daily. I know that you don't want her to get dehydrated again, but you may be trying to get her to drink too much. This is why you should make sure you set a realistic goal of how much she should drink. Next, try to get her to drink liquids that she enjoys. If she has never liked to drink plain water, then try sodas (if you are worried about sugar, try diet sodas), juices, decaf coffee, tea, or even mineral water. You could also try alternative to drinks, like jello (which turns into a liquid in the stomach), milkshakes, soups, or ice cream/ sorbet/ popsicles to fill her need for liquids. You may find that she likes these alternatives better.
Now, look at the way you are serving the liquids. Many people don't do well with straws. You can try to give her smaller sips from a cup of liquid instead. Or, she may do well with the liquid on a spoon.
If your mother was the type to never drink much her whole life, please understand that you will not change that behavior now, no matter how hard you try. That is why the alternatives listed above may work better. Other thoughts I have about fluid intake is that as older people reach the end stages of their lives, many times their urge to drink really diminishes. She may feel like you are forcing her to drink when she really doesn't want to. Another thought I had was that sometimes older women don't want to drink much because they are afraid they will be incontinent, especially before bedtime.
If my suggestions do not help, please talk to her health care provider about any further ideas that they may have to help you. Good luck!