If you don't know what their disability is, then you can't possibly know how it impacts them and what can help them. Saying you 'don't think their disability prevents them because x y or z' is uneducated at best, malice at worst.
It's not for you to decide what they can and can't do, it's up to you to either agree to a reasonable request, or not. I think what they're asking to do is perfectly reasonable, and won't prevent you from, well, anything. Just because you feel it makes things a bit more hassle for you, is not a legit reason to refuse to reasonably accommodate them.
For example, I have m.e/cfs. One common symptom is brain fog. Unfortunately for me, it's quite severe, has gotten worse over time, and severely impacts me. It means I struggle to take in new information and learn. As a result, at the grand old age of in my 30's, I can not for the life of me get to grips with cloud, dropbox, data sharing etc, and as a result of not understanding it properly, become very anxious I'll share something I haven't meant to, with people I shouldn't. I use email as I understand it well (having learned it a long time ago as a child), and can use this without a struggle. Trying to figure things out that I can't understand on a computer, takes me a very long time, and leaves me with migraines, and flares up my exhaustion levels - and I'll need to do it all again the next time because of the impact of struggling to take in new information.
This would not happen with a person who does not suffer my disability. Therefore, being able to send it via an email instead, prevents a flare up of my symptoms and would be a very reasonable request to accommodate. To have to do it the way you're requesting would leave me at a disadvantage (ie: it's known it flares up my condition and will make me poorly).
A reasonable adjustment for a disability doesn't mean a person can't do something. It means that the adjustment will help them do it easier, and have access to do the same tasks as others. In your case, sending the receipts is the task. How they do it is inconsequential to you, you still get the receipt, but probably consequential to them in how it impacts them. Not everybody who has disabilities is trying it on. Most of us absolutely hate it, and even worse, when people not only don't understand but refuse to try to understand.
Lastly, I've no idea what disability they have and it doesn't matter. It's your sibling. Stop being a dick. No one should have to give you these examples to get you to understand disabilities and why they can do y x or z but not something like online sharing.