If someone connected to my WIFI could they get information about my wallet or steal from me?
Just getting connection to your WIFI would not instantly allow someone to see your wallet information, gaining access to your WIFI is only allowing first and foremost access to your internet connection.
However, someone with access to your WIFI could essential hyjack your information, if a few other things fall into place first.
If your WIFI router is set up as a Home/Work Network and Network sharing is turned on, then a hyjacker, could potentially view any files/ documents shared on your network. Your computer would have to also be set-up, to share files with the Home/Work Network, therefore, if for whatever reason you stored your wallet information on a non-encrypted file, then the hyjacker would aslo be able to access this information.
Another method to gain access to your wallet data is through: packet-data sniffing; if a hyjacker has gained access to your WIFI, and they have hacking knowledge (pretty basic hacking knowledge in all honesty) then it is possible for the hacker to "sniff packets" of data that you send over your WIFI, if at any point you send your private wallet data over the WIFI, then it is highly likely that the hacker will be able to see your private wallet information that you have sent through the WIFI.
To ensure your WIFI is secure:
Ensure your router has a Strong and Unique WIFI password.
Ensure your router firmware is upto date.
Enable WIFI encryption WPA2 if your router is newer than 2006, else WPA should be adequate but less secure
Change your WIFI SSID (Name) to make it less easy for the hacker to directly relate the network to yourself and/or allow the user to identify the brand of router that your network uses.
Getting access to your router is the first step towards your computer so use a strong WPA2 password. Also use a different admin pass and a different log in pass. The main concern with someone accessing your router is packet sniffing, which means that person could follow your non https traffic. This does not affect your wallet.
Once someone is in your router, however it is easier to exploit your computer if you have vulnerabilities such as not up to date and patched OS. Regarding your Monero wallet, if you have a strong password for your wallet/s, even if someone accesses your computer, they will not be able to open your wallet (to spend or see balance). However, if they do manage to access your computer, they could install a keylogger and grap your password when you open your wallet.
So, I suggest you use Linux Os as it is much less vulnerable than Windoze and way fewer malware. You also need root to install anything which adds an extra layer for you. Then make sure you have a strong password for your router and strong password for your wallet (all different obviously), and you should be fine under normal circumstances. Oh, and make sure you haven't labeled any parts of your computer for local sharing.