For many older/elderly travelers, it's tough to ask them to visit only for 2 or 3 weeks. IME, it can take them several days or almost a week to recover from the jet lag, travel exhaustion etc. When my MIL visited from overseas, we didn't realize how much recovery time she would need and it caused us some grief. Even if your MIL is fairly young and robust now, in the future it will become an issue.
2.5 months is a long visit though. I'd plan for 6 or 7 weeks, and perhaps consider short-term rental accommodations. Perhaps the next time she visits, your MIL could do a home exchange with someone in your community who wants to travel to India.