Would it be okay to tell my sister in law to stop saying her stillborn baby matters more than her living children?
Why would you say that to someone who is grieving?
Because I really feel she should start fresh instead of focusing too much on a deceased baby.
She has been through a traumatic time, so make sure you are being sensitive. You could add more hurt during a bad time. Show her love and support, and make sure you are tactful, if you have to mention it.
I am a tactless person and I will never be able to change.
Everyone is capable of change, but you have to be willing to think differently. Pay attention to your sister in law's emotions and show her your compassion. Be delicate with her situation so she can grieve properly.