I can't advise on Ontario law.
I think in Alberta the niece would be out of luck. Only a spouse or child can challenge a Will (assuming the testator had capacity, and there was no undue influence in the creation of the will).
Alberta statutes:
Wills Act:
http://www.qp.alberta.ca/574.cfm?pag...cln=0779726898
Dependants Relief Act:
http://www.qp.alberta.ca/574.cfm?pag...=9780779721511
Adminstration Of Estates Act:
http://www.qp.alberta.ca/574.cfm?pag...=9780779734900