My mother ( 45 ), who has been separated since 2018 from my father, is dating a guy who is cheating on his spouse and has three children, out of which one is a little physically disabled.

Good that you told your age. Learn some skill, with age by your side I'd suggest learn to code and get a good job. It might be harsh but without this particular thing, nobody is really going to give any weight to your opinion and/or priorities. You can do much as an independent adult, and much less as a dependent child. Before you solve other people's shit, they have to believe you are somebody who commands that sort of authority. Again that sounds orthodox but in most practical situations serves well. Give yourself 6 months to 1 year and you will be heard. Also, if things do not improve on the personal front and start getting uglier, being in a stable profession will hold you in good stride. Trust me. All the best.

/r/delhi Thread