You replied to my response to him which is why i responded to you.
Please refrain from giving wrong advice if you do not understand the situation in the first place.
He has not got a medical request for his sister because she is not eligible to be sponsored!
You are allowed to include any dependent children of your parents in the application as long as they are under 22 years of age and you are able to meet the income requirements. This info is clearly stated on IRCC's website.
The Medical Request for my sister came in the next day so the matter is now resolved. Thank you all for your help!