Do I need to be concerned or do I leave it be?

[Wednesday66]

My LO will be staying with my in-laws when I go to work. They also look after my 5 year old niece (by marriage). My niece is an only child and its pretty clear that she's never been taught how to share. If my LO plays with her toy she gets mad and sometimes gets in the baby's face and tells her "It's mine! You can't have it." etc and snatches it away. But my niece will also take my LO toys from her and play with them, even decides that she's going to take the baby's toys home at times (I don't let her do this). Obviously the snatching and abruptness startles my LO sometimes and she gets confused and upset.

I feel like it's not my place to teach her how to share or say anything but I don't like how she is with my LO. I'm also concerned this behaviour will rub off into my LO since they'll be spending a lot of time together.

It will be for your in laws to deal with, not you.

There will come a time when your LO is a sassy youngster not wanting to share too. You don't need to judge, they all go through it.

Unless there's a huge backstory here with significant favoratism towards your niece, then part of trusting your in laws to be childcare means trusting them to manage turn taking and sharing.

Your niece will be in school for a large part of the day presumably?
Also would it be an option to look for other childcare - nanny or childminder or nursery if you are very concerned?

Tricky, but hopefully the ILs will deal with it when in charge