My sister will be in USA for 100 days with Visa Waiver. Is there a way that she can travel to another country and enter the USA again, so as not to contravene the 90 day limit of Visa Waiver?
Your sister cannot stay legally in the US more than 90 days with the visa waiver program. Nor can she go to Canada, Mexico or some nearby islands as the time spent there will be included in the 90 days, specifically to address the visa-run problem. Even if she flies to, say, Europe, and returns quickly she risks refusal at the border.
If she wants to stay in the US legally for more than 90 days she should apply for a B1/B2 visa which will allow her up to six months.