PPR relief is granted on a time basis, so moving into a property for just 6 months will not work to completely wipe out your CGT liability on any gain.
However, you can claim PPR relief on a property which is occupied, rent free, by dependent relative or a widowed parent. So, if your mother has lived in your second property rent free for all the time that you have owned that house (and she was either widowed or she was a "dependent relative" for that period) then you won't have a tax liability.