Rossei said:A visit visa (TRV) seems perfect for your case. However, Super Visa is easier to get (not applicable for your sister though).
If you decide to try TRV first, write a good invitation letter explaining that your sister has a govt. job and your parents want to travel together with your sister and they cannot stay here for more than 3 - 4 weeks. Do not mention 'months' when you're applying for TRV. All their financial status seem good enough. I'm not sure though how your grandfather's property can help in this case.
Thank you so very much for your guidance. Do you happen to have invitation letter template? If yes could you please forward that to me?