Its very easy to do.
I just do in a find replace window that excepts regular expressions
if "brother" is the string your looking for heres an example.
Replace ^.*brother.*\n with nothing ( blank)
that wont remove lines without the "brother string"
so...
1 2 3 brother 3 2 1
4 5 6 sister 7 8 9
will reduce to
brother
4 5 6 sister 7 8 9
which then becomes the problem as originally stated.