Help with relationship

Okay so stupid problem but basically I want to go out with my girlfriend on Valentine’s Day, but I haven’t told my dad about her (I’m gay and it’s awkward to tell him :/) and whenever I say I’m going out with friends he asks if my sister is coming too since we have the same friends. So if I go out on Valentine’s Day without my sister it’ll be REALLY obvious I’m going on a date. I’m 16 by the way if that’s relevant?

So does anyone know how I can get out of the house without my dad knowing what’s going on??