His mean is if your mother die in future and your sister will orphaned then you can directly sponsor you sister , but your mother is live and if you want to sponsor your sister only on PR then its not possible, means you must have to sponsor your mother and sister both togather.
1) you can not sponsor your sister only on PR when your mother live.
2) if your mother die and your sister is orphaned then you can sponsor your sister directly.
3) when you sponsor your mother and sister lets assume both are get PR and both are in canada but your mother dont like to stay in canada then she can leave canada and your sister can stay with you. in this case your mother lose her PR but your sister will not lose her Pr.
I think you will understand point...
OMM
1) you can not sponsor your sister only on PR when your mother live.
2) if your mother die and your sister is orphaned then you can sponsor your sister directly.
3) when you sponsor your mother and sister lets assume both are get PR and both are in canada but your mother dont like to stay in canada then she can leave canada and your sister can stay with you. in this case your mother lose her PR but your sister will not lose her Pr.
I think you will understand point...
OMM