I been having the urge to not tell my insecure girlfriend that I'm letting my ex-girlfriend stay at my place for a while. My ex-girlfriend's stepdad kicked her out.
Why would you hide that from your girlfriend?
It's none of her concern. Moreover, I don't want her to get jealous and upset
That's really wrong to lie to your girlfriend about your ex. She has the right to know that, whether she gets jealous or not.
Her opinion shouldn't matter -- she's annoying as they come.
Of course her opinion should matter, you are in a relationship with her. If you feel you cannot be truthful with your girlfriend the maybe would be best to break things off. You need to be honest with your girlfriend about what is going on.