Hello Mary,
I am currently a student in college and we covered this topic the other day. The bottom line is whether or not creating an LLC with enough assets can protect you and your sister as individual/partners. Many if not most banks will require you to co-sign a loan along with your LLC so that there is some consideration coming from the person/business entity receiving the loan.
The reason the person said don't bother creating and LLC if you have no assets is because you will rarely get a loan signed by solely your limited liability corporation (LLC). If personal protection is what you seek, you would need to build a decent asset base in order for lenders to feel comfortable enough to lend to your business.
With all of that said, poor business practices can make you liable for repayment to your lender even if you are "protected" by an LLC. I am not totally sure what "poor business practices" entail but I can imagine a big bad bank would do anything in their power to get their money back on non-performing loans.
I would say your best bet is to talk to an attorney. I'm sure there are some here on BiggerPockets that you could connect with and receive a more professional opinion.
Good luck with your endeavors!
Howell Conant