visiting visa for cousin

dear experts ,
could anyone help regarding my cousin visit visa application.

I'm on tier 1 entrepreneur visa, visa expires on 11/04/2017 going to extension on 1st week of April.
my wife was pregnant, so decide to bring her mother & sister.

my question is

1) can i apply visit visa in march for them?
2) should i explain anything in covering letter?
3) if i cant apply, any other option to bring them for 5 months?

please advice

thank you

regards
svs

Be careful hete.

5 months is more like residing than visiting.

No work permitted on a visit visa, not even unpaid childcare.