How much does cosigning on a vehicle affect you?

I am 27 years old, and have just cleared myself of all debt other than my truck payment and mortgage payment. I have paid off 4 vehicles before this, but this is my first mortgage. My credit score is hanging between 795-800. I would like to look into investing in some properties or rent houses, to better me in the long run. Issue is, my sister is looking at replacing her problematic car and buying something a bit nicer, and wherever she goes they tell her she must have a cosigner. She has no issue paying her payments, never really has, just not enough credit history. I wouldn't mind cosigning to get her into a good car, but I would like to know how much this will affect me and my ability to buy something like a rent house or property?

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