I need help ASAP, I need answers :((

Okay so I’m 20 and live with my family still and my cousin(22) lives with us since she was like 4 and she’s basically a sister, anyway, the thing about her is that she’s very small minded, she yells at anything that she doesn’t like, maybe she’s mental but idk, so 2 days ago I’m pretty sure she has a cold sore (herpes), she has a baby who’s almost 2 and she’s my whole world, I love her so much and I want what’s in her best interest! Anyways, back to the cold sore, I’m pretty sure she’s been having it for maybe 4 days and like she doesn’t wanna go to the DR to check it out bc we don’t know if it’s a cold sore, she won’t listen to me bc she’s mad at me from an argument we had a week ago and she’s ignoring me, yeah you see how she it? Anyway, my sister told her to not kiss the baby or share anything with her until we find out what it is and my cousin said, and I quote, “ I could do anything I want to her bc she’s my daughter”, like what the fuck? I don’t know what to do I’m so stressed bc she won’t listen to anyone, what do I do? Please help, and sorry if this is all over the place, I’m worried.