From my take of it Uma befriended Ploy the maid as she felt lonely with her husband always busy working. Since Uma had a hard time getting pregnant and people questioning her and the husband about when will they start having kids. She and Nirach decided to use Ploy to make a baby and not tell anyone about it. Overtime Uma got jealous that was the only thing she couldn’t provide for her husband and when Ploy slipped on the bathroom floor and hit her head and “died” that was the perfect moment to get rid of Ploy the maid. As for Joy she was cray cray to begin with after killing her previous employers 😂