THIS IS SO RANDOM BUT I NEED OPINIONS

So i’m 22 years old i live with my sister me. So my sister has been like curl to me she been a punk When i was about 19 i wanted to get a car but my credit wasn’t as good maybe like 490 when we went to the dealership to see what i would get they told me right off the bad i need a CoSigner she told me she wasn’t gonna risk signing for me so we left and i didn’t get the car Now i’m 22 i have a 700 credit she want to buy a house and now i want my car and now i can actually get it without her but she wants me to help her I really thought about it but my Bf told me that if i sign with for a house that it affect me later to get my Car i won’t be paying for the house just to sign He said it would still mess me up that my interest rate could go up bc the will see a house on my name WHAT DO U GUYS THINK WILL THAT REALLY HURT ME ?