If you are illegal without work authorization and your brother in law knows it and is an employer, he can be subject to employer sanctions for "knowingly" hiring an "unauthorized worker." So, if he files papers to sponsor you and you are working for him while the process goes on, he can be fined if the paperwork triggers an audit of his company. This is because the initial set of forms ask for your current status and work history. If he does this regularly with others, he could also go to jail for a "pattern or practice" of hiring workers without authorization. If his business is raided and you are caught, you can be deported. The process to sponsor an employee, whether legal or not, is complicated, and normally requires advertising and recruiting for US workers first. Assuming there are no qualified US workers for your occupation, then, depending upon the minimum requirements for the job and where you are from, you might have to wait many years for the quota to become current before you could even fill that job lawfully with the green card. So, why would the employer hold a real job open for so many years? There isn't enough information here to tell you how long that would be or what would be required to sponsor you. But, it sounds like you would be out of status during the process and subject to deportation throughout it unless we get some kind of legalization program passed by Congress in the interim. Doing favors for relatives in this situation doesn't always work out.
If you are out of status, you cannot secure a working visa - in fact, if your brother-in-law tried to sponsor you for a work visa, if could raise the fact that you are here illegally and result in you being placed in removal. Being here unlawfully for 7 years has grave consequences and if you are removed, you will be barred from returning for 10 years.
