Since you are not paying any mortgage,I am guessing that you are not.
You should speak with a mortgage consultant or your friendly bank loan officer.
If you are a passive owner (minority shareholder as tenants in common) with your sister/brother in law, you might be able to qualify as a first time buyer.
If that is the case, then providing that you can qualify for a loan to cover the other 50%, then it shouldn't be a problem. That is provided your sister and brother-in-law are agreeable, of course.
You should talk to a good local mortgage specialist to check out your likelihood of qualification.