Vinnie's illness

The boys (Clarence, Jr. and John) put some of the patent medicine they were selling into her tea, making her extremely sick (the stuff actually kills one of the neighbors' dogs, which is how it's discovered that they gave some to Vinnie)...

Father: You're going to every house where you sold a bottle of that concoction and buy it all back.
John Day: But it's a dollar a bottle.
Father: I don't care how much it is. Here, I'll give you the money now. How many bottles did you sell?
John Day: A hundred and twenty-eight.
Father: A hundred and twenty-eight!
Vinnie: Clare, I always told you John would make a good businessman.
Father: Young man, you'll have to come down to my office with me. I'll give you the money to buy back that medicine. $128, and $10 more for Mrs. Sprague's dog, that's $138. But it's all coming out of your allowance. That means you'll not get another penny until the whole $138 is paid up.
John Day: [does math on his fingers] I'll be twenty-one years old!
______________________________________
"Leave the gun. Take the cannoli."

But she was sick before the boys put the medicine in her tea.

I think she just had a headache before, and then the stuff they put in her drink made it worse.

I doubt if they were working with Vinnie. The second doctor didn't even know the family. To ask busy doctors to leave their offices and patients to play a trick wouldn't make sense. Vinnie was extremely embarrassed about Clarence not being baptized and she wouldn't want anyone but the minister to know about it, least word get out.

She seemed to have a mild flu, then got much sicker after the boys poured the snake oil nostrum into her tea.