Serious Concerns about Security on Network (Wired - Ethernet CrossOver

Ok, just recently networked two computers - via straight ethernet crossover between the two (no router).

Both computers talk to each just fine. Quickbooks is installed and set up for multi-user mode so that information entered on computer b writes to the data file on computer a.

There is also an internet connection - no wireless but cellular via a hotspot - that's separate obviously but both computers hit this hotspot in order to connect to the net (mainly to check email, get info from vendor websites, etc.)

There is nothing special enabled on the network or computers for security other than norton.

Now, here is my question - is the data on the host computer at any additional RISK due to the connection to 2nd computer? Or is the risk the same whether they were connected or not. My concern is that obviously the data files on the host computer have been "shared" and by shared I mean shared with 'everyone' (QB seemed to think that was the only way to set that up).

I thought that shared with "everyone" just meant "any other computers" on the wired network - which , there is only one so that's not an issue.

But does sharing that folder with "everyone" in any way open up that data to someone just sniffing around = externally???

Thank you so much for your advice!!