Storage is a decision a family makes once and then pays for every month. Before you sign, the question is not which facility has the nicest office. It is who the contract binds, who gets in, and what the facility is allowed to do if the rent falls behind. Those three lines decide whether the unit stays a shared asset or becomes the source of the next argument.
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Check the name on the agreement
Read every name on the rental agreement. If your mother is the tenant, she needs to sign, and her name stays on the account even if you handle the payments. If you sign as the tenant, the unit and its contents are your responsibility on paper, whatever the family agreed at the kitchen table. Ask whether another person can be added as an authorized user, and in what form. Do not let the contract go out with only your name on it when most of what is inside belongs to your mother or your siblings.
Check the access and the gate code
Ask for the access hours, whether there is a gate code, whether the code is unique to each tenant, and how a new person is added or removed. Find out whether anyone in the family can reach the unit in an emergency without you there. Write down who holds the code and the key, and put that list in the family record.
Read the missed-payment clause
Find the section on late payments and on what happens after the account falls behind. Ask the facility, in writing, how much notice it gives before taking any action involving the contents, who receives that notice, and where it is sent. The notice address matters. If the notice goes to your mother's home and no one is checking the mail, the first warning may be the last one. Ask whether the facility can send notices to a second contact, and add one.
Compare the unit size and the exit
Measure what has to fit before you choose a size. Get the unit that fits the boxes, not a larger one that becomes an expensive habit. Ask what the move-out process requires, including any deadline to clear the unit and whether a final walk-through is needed. The exit is part of the contract even though it feels like a problem for later.
The Start here page explains how to keep decisions like this one in a single place where the whole family can see them.