It's a dismissable offence to look into anyone's account that does not directly involve your current application. If it is a family member, they are required to make it known to their team leader and not have anything to do with it.
Your SIL could call in an anomymous tip off if she suspects anything.
Yes they do system checks regularly and staff dismissed due to that. The staff are not advised directly of what is involved, just an email reminder of the values they agree to.

If you suspect anything that may sound amiss, then you should report it and it will get investigated.