If an elderly woman, suffering from dementia, was killed by her 54-year-old alcoholic son, we can be sure he would serve a lengthy prison sentence, and his alcoholism not considered a mitigating factor. And if the killer was her 54-year-old alcoholic daughter? 16 months:

http://www.stokesentinel.co.uk/Alsager-alcoholic-Ann-Lupton-jailed-killing-mum/story-20498657-detail/story.html