with all due respect tippy, what I understood from Sally's mom is that she cannot find a similar car to rent. All the insurance company can do is provide payment for a rental, they cannot produce a rental vehicle that is not available at the rental company.
Since I didn't have all the information on why she was unable to find a Sienna, I was offering some suggestions on how to get something that wasn't too small for her needs and also offering alternative cars in the same niche that she might also like. Given that she may be without a personal car for an extended period while she evaluates whether to repair the current Sienna or to try to find or order something new, she shouldn't have to live with an inadequate vehicle in the interim.
On the basics of what an insurance company can do, though, that depends on what her coverage is and who is providing it. If she has a flat daily fee for rentals from a large provider, she can shop around for rental companies who can procure her what she wants or something comparable. If it's not available today, she may be able to swap out if they can find and deliver the right thing for her in a few days.
If she has an independent insurance agent, she can ask them for their help in locating what she wants and making sure the coverage applies. That might mean going through a series of rental companies on her behalf. Independent agents will often take an active role in helping you recover from a loss and in accessing all of the coverage you've paid for.
Without in depth knowledge of her insurance plan and the local rental agencies, I was only able to give general advice.
No particular respect due here, though I did work briefly for a large car insurance company.