Am I being unreasonable here?

The story is this:

I am on food stamps.  Well, not actually food stamps, the program that replaced food stamps.  It involves a card that can only be used to pay for food but otherwise operates like a debit/credit card.

When someone gives me a ride to the store we check out separately.  Part of the reason is that it's flat out simpler.  If we each check out separately then it is completely clear who has to pay for what, and thus there's not calculation or confusion.  Part of the reason is because I can't pay for other people's food.  It's illegal.

Now neither I nor anyone likely to take me on a trip to the store is going to try to get around that law, but if we were to show up in a checkout line, put all of our stuff together, have the other person pay a certain amount in cash, and have me pay whatever is left over... well it looks like I'm helping them buy their food.  At least it looks that way to me.

If I were working the checkout line and one person started paying, ran out of cash, and then another person put the rest of the order on a card, my guess would be that the second person was helping the first person pay for the order.  And if I knew that it was illegal for the second person to help the first person pay...

With all that in mind, my sister suggested something where we could use some coupons she had if we put our stuff on the same bill, she'd pay her part in cash, I'd pay my part on my food card, and then all would be well.  And I asked her to explain to me how this was supposed to work in a way that didn't look like we were breaking the law.  To be clear: as long as the portion of the bill she paid was equal to or greater than the actual cost of her stuff it wouldn't be breaking the law, but it really seemed like it would look like it to me.

So I asked how she planned to have it not look like we were breaking the law.

She got angry.  Well, actually, first she acted like I was an idiot, then she got angry when I tried to explain my concern.

My concern, for the record, was not that the checkout clerk would get on the hotline to government investigators who would in turn throw me in jail.  My concern was that I get uneasy doing things that potentially give off, "I'm doing something wrong," vibes.  So even though it would be legal, and even though bad things were unlikely to happen regardless of how it looked, I didn't want to do it unless it would look as legal as it actually was.

She never did explain.  The result is that it is not going to happen, and she is pissed off at me now.  That second point is clearly because she thought I was being unreasonable but I don't think I was.  I'd like a second opinion though.

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And now I return to writing a post on Godzilla versus the Sea Monster, already in progress.