hi, it depends on which parts of the house are grounded properly, in this case im not so sure your sink is, I would first suggest you consult and electricain, although failling that, you need to bond the two pipes under the sink, and make sure there is an earthing conductor connected to you incoming water mains.
To be honest though, if you suspect missing earths in your home, especaily you shower, you need an electricain round there as soon as possible.
It could simply be down to chance that you have not been killed by the shower (worst case senario obviousl) but I would recommend having it seen to imediatley!