You can leave Canada without a PR card and your sister can mail or courier it to you as despite a few posts on here although discouraged by IRCC it is not illegal.If there is any risk at all it could be with a receiving country no way to predict but again nobody seems to have indicated this being an issue on here at least, mainly rumours.
There is no such rule that you need to be in Canada 6 months every year, all you need to do is to ensure that you accumulate 2 years in the first 5 years post landing. After that the 5 years becomes a rolling 5 years on each entry so you need to have 2 years banked in the 5 years prior to each reentry into Canada.
In effect you could stay away for near 3 years post initial landing but that would mean then staying put for 2 years without leaving at all to meet the residency obligation..
Also note that date of landing is the date for the initial 5 years and not the PR card issue/validity date.
Generally new immigrant PR cards seem to have been issued in 6-8 weeks so you are borderline to get yours before you leave the country.