i think your mother should get a u.s. visa...she is a p.r. and not a citizen of canada yet...
That's what the OP just said:
Few days back my mother got the immigrant visa for Canada. She will be coming to Canada probably next month. My mother wants to go to USA first(has US visa)
to visit my sister and then come to Canada from USA after few days. Is there any issue with this? Instead of coming directly from India she will be entering Canada via USA.