Hello L-General,
Interesting set of circumstances you have there.
My advice would be to only do what they have asked of you - Send an updated Schedule A form (with details from age 18, right?)
You are at the Medical Stage, which means your application has been processed.
Any changes and amends will delay your application, because then verification will start again.
If you are not in touch with your half-sister, do not make any mention of her.
(Family is asked to be included, so that later no one can claim to have family members who want to come to Canada.)
Sit tight, you are almost through.
Do the medical, send the documents and in 2-3 weeks, you should receive your PPR.
Best of luck,
S