In order to do a short sale, you must complete financial disclosures listing all of your assets. You must list any acct or property where your name appears as a legal owner, even if it there just for convenience. If you do not, the bank may find it anyway, and determine that you willfully misrepresented your financial position.
You can try to explain to the bank that the account is "Mom's" - but they will likely look at that as an asset of yours. In the future, you can always check with the bank about putting you on Mom's acct as POA instead of a joint account holder.
I have closed short sales where a seller owned other property. The lender must ascertain that forgiving the debt, or working out a post-closing payment plan for all or partial is the best business decision for the bank. The bank is not there to "help" you - but rather make the business decision that results in the least pain for the bank. If you really cannot afford your payments, it is in the bank's best interest to work out a solution. If you have other assets that are not liquid, such as real property, they would want you to sell it. Since you cannot control that property as a minor interest holder, you will need to explain that in your hardship package.
I have closed short sales where the bank forgave debt when the seller owned other property......so it can happen. It has to make sense, though.