I would put my finger on the end of the straw and dribble water into Mom's mouth. The hospice nurse actually advised against that because they thought she would aspirate fluids. They gave us these swabs we called lollipops which were sponges on the end of sticks. We used them to keep her lips moist. It was just to keep her comfortable. Towards the end, she stopped taking any fluids or food.
Since you say she is on hospice, what does hospice suggest? Is your mom actually thirsty? A mouth sponge dampened with water can be used to wipe her mouth out. Trying to get her to drink if she isn't able to swallow can lead to aspiration pneumonia.
My mom was occasionally having trouble while she was at home but with a shorter straw and a little coaching we usually were able to make it work (in her case it seemed more that she forgot how to suck on a straw). Since mom has moved to the NH there is no one who can take the time to encourage her so all her fluids are made honey thick and spooned into her.
Ya know, if I were on hospice, I'm not sure I would mind having aspirational pneumonia. I think I'd rather risk that than be thirsty. BUT if I were at a point of not wanting to drink, then the comfort measure of something to keep my mouth moist would probably be better.