@Laads- It should not be that hard. Most important thing is that your MIL has to prove her ties back home and that she has family, property and connections that she have to return to. My mom had her interview few days back, my SO invited her. . They just asked her four questions.
1. Reason for visit- family visit
2. Her and my dad's employment status
3. Her property, amount of land.
4. What my siblings do.

She was telling me that they repeated the same questions around just to make sure my mom was consistent with her answers.

Chances are doubled if your MIL applies alone that together with your FIL.

Good luck.