In reality Daisy, Lily and Violet were always portrayed as carefree, incompetent trainers not caring much about taking care of gym, training pokemon or following procedure.
They were successful in performances, underwater ballet etc achieving big popularity being famous but in battling they never really showed much interest neglecting their chores.
Than when opportunity arise they decided to slip away from gym responsibilities and force Misty to go back interrupting her quest of becoming water master. Even when she cam back they still never helped her much with Misty being frustrated with their irresponsibility.
Although out of three at least Daisy spend some time with her at gym helping out, and she was the only one who showed somewhat of interest in battling if Luvdisc episode was anything to go by having potential to become decent substitute one day if Misty leaves gym. Which isn't too farfetched to expect given her desires were always to be something more always striving to become strongest water trainer in world being of adventurous spirit. Judging by Gold and Silver it seems same line of thought can be applied to game counterpart too which expressed desire of going on journey to become stronger.
Misty's sisters were always terrible, unless you're talking about the gym Misty as a trainer was portrayed as average most of her run. Her Poliwhirl was obviously her strongest then Corsola was okay/good, everything else was pretty average to weak.
In your opinion anyway. From what was showed Misty was actually pretty skilled trainer having several noteworthy achievements if you ask me and aside from Politoad and Corsola, Staryu, Gyarados and to some extent Luvdisc were either strong or pretty good. On Psyduck i cannot comment because he can be both useless and powerhouse depending on circumstances.
