how to tell my mom?

So my dad passed a year ago December 27th, and I am moving out with my boyfriend next month. i’m scared to tell my mom because my brother and sister are 35 and 30 and still live at home and she never had anyone move out before i dont know bow she’s gonna react and i’m scared. my boyfriend thinks I don’t want to move out with him because I haven’t told her yet but I’m very scared to do so is this normal? I’m 21 years old and the “baby” in the family. I’m only moving like 6 miles away to an apartment with him but I dont know how to tell her. Advice please!