Here is what my friend did when she had to return to work and had to get her mother-in-law to come over to play the role of the primary caregiver to her baby:
- Introduce them and gradually increase their exposure to each other
Her in-law will be around whenever she feeds, bathe and change her baby. Her mother-in-law would also hold the baby (for as long as the baby allow) everyday. Slowly, MIL started taking over some tasks while my friend stood around to make the baby comfortable.
- Left baby with MIL
Starting with very short intervals, my friend would leave the room while baby and MIL gets some time on their own. If baby starts crying, she would wait a couple of minutes (not too long) before going in. Slowly, her baby got used to her MIL and learn to trust MIL as another caregiver.
And like what Vanessa mentioned, she also kept explaining and reassuring her baby throughout the process. Took quite awhile for the transition but most important thing was that baby eventually got comfortable with her MIL.
Hope this helps!