The reason it has been said to not shower or wash hands etc during a thunderstorm has to do with metal piping and the lightning.
The lightning could strike the metal pipes and then send a charge of electricity through the metal pipes and into the water that comes into contact with your skin.
That would then lead to being shocked and possibly electrocuted from the lightning.
But most homes especially new construction homes use plastic water piping which cannot be struck by lightning and besides water pipes are buried under ground outside that leads to your house so they should be protected from the lightning.
I've taken showers and washed my hands and did the dishes as well during lighting and thunderstorms and never have I been shocked.
Your more at risk of getting electrocuted by lightning when walking outside in the thunderstorm than you are when washing your hands or taking a shower.