Living Separated from husband, visa for children from India

I am posting on behalf my sister, who is currently in UK on tier 2 visa. She is living separated from her husband for over 3 years now, 2 children back in India. Now wanting to bring them over here, does Indian immigration or UK visa require any affidavit from husband as they are not divorced yet.

Where does the father live? Do the children currently live with him? Are they legally separated (a decree of separation from a court)?

Father is in India, no contact for 2 years now, kids lived with mother p, now with grandma as sister got a skilled sponsor visa.