Extending visitor visa for mother

[wadheraswati]

My mother came here in mid Feb for a 6 month stay and she has her return ticket scheduled for end of July but we want to apply for her extension to look after 2 year old daughter. A few people have told us that we cannot give that as a reason to IRCC. What is the best reason that we can give that will increases the chances of getting extension. Please share your experiences.