Baby isn't turning away from you because she loves, or even likes your MIL more. My LO does that at day care and I know that she doesn't like her teachers more than me (or at least I hope not lol). I've come to realize that it has to do with the transition from day care to home. She is excited to see me, but has been at day care all day so that is where her mind is at. Once we get home it all goes back to normal. I think that is normal for a lot of babies, so it could just be that your LO was in MIL mindset. That being said, your MIL should not do that at all and I'm sorry she did.
Reply to Ugh worst nightmare - baby turned away from me and into MIL arms