For a baby to learn to walk, she must first develop months of muscle building, balance and motor skills. Your baby will achieve new milestones on her own timeline. You cannot teach her to walk, but you can create an environment and provide encouragement to help her learn.
Most babies start walking between nine and 18 months. That's a pretty big variance. Just because your sister-in-law's baby is walking at 10 months, doesn't mean yours is ready to, and it doesn't mean there's something wrong with her.
Now, you did tummy time, right? She loved that. Then she learned to roll over, then sit up. Now she's pulling herself up. This is the time to encourage walking.