Well, your mother should get to choose the name for her own baby. You might not like the name but you will have to get used to it. When you have a baby then you can choose whatever name you want and she will have to tolerate it.
I know a girl called Indigo and I always think its more a boy's name.
I know a little Indigo and it really suits her. However it was the parents choice and took them 3 weeks to get there and her mother hated it
If you aren't convinced, then ignore MIL, no-one else should be attempting to name your child. If baby isn't here yet, just saying you're waiting to see what she looks like. I know loads of babies who didn't look like the name picked out for them, so ended up with something else.