How do I prevent connections? When on my home WiFi network (or really any open network), when the WiFi drops or lags my phone automatically connects to my sister's phone (or that oepn network's 4G/5G alternative).

For example:

Our WiFi name is NetGear77. When it kicks me off NetGear77 due to instability or not loading fast enough, it forces me to connect to NetGear77-5G, which is my sister's phone.

My question is, how can I stop my phone from connecting to the other network when the WiFi lags or drops? It is weird when I can see my sister is searching up stupid crap like Twilight. I already have mobile data turned off, but I don't think it does anything.
Remove your sister's WiFi network from your list of APs. 


Settings-> WiFi
Long press the network name, which should be "NetGear77-5G"
Select "Forget Network". 


Also your sister probably should not be running a WiFi access point all the time. This will reduce her battery life, among other security issues.