My mother would also vote Democrat, no question. She despises the GOP.
I think a large part of it is the right's deep affiliation with religious groups and organizations. My mother was studying with JWs in the late 1960s and was baptized in 1977. Not long after, Ronald Reagan became president and the Repuplican party openly courted religious organizations. For JWs, that meant the GOP was in bed with the whore of Babylon, false religion. By itself, this would have made it very unpalatable to vote Republican.
Also, we were poor and dependent on government programs. Reagan was famous for making an example of the "welfare queen" and promoting self-reliance and self-dependence. My mother was like many JWs, scraping by on low income and state handouts. Democrats were promising to protect those programs and even expand them; Republicans were complaining that those programs were already too generous.
In other words, it wasn't the entire party platform that JWs were voting for. Democrats support abortion, gay rights, and are seen as undermining religious authority. These all would be big red flags for a JW. But they also support welfare programs, broader health-care coverage, unions and worker's rights, progressive tax systems, and so on. All of these will appeal to low-income families, who wonder how they can be struggling so badly in the world's wealthiest country.
I could imagine her holding her nose as she voted Democrat. But she would just as soon cut off her own hand than vote Republican.