There could be many reasons, so let's first start with a few definitions.
Wifi speed refers to just the connection from your device (e.g. laptop) to the router. From your question it sounds like the modem includes the router, which is pretty typical. For simplicity I'll simply refer to the device as the router. Wifi relies on a strong signal from the router. Signal strength is very dependent on the distance you are from the router. The number of walls (and their composition) or being on different floors can also affect signal strength. You can get a free wifi signal tester for your phone.
In addition, there are different frequencies for wifi. The older, slower frequency (2.4ghz) generally supported speeds up to about 20mbps. This frequency can also get interference from household appliances such as the microwave. The newer frequency (5.4ghz) supports up to 200mbps, has better range, and supports more simultaneous devices. If your sister's modem does not support both of these frequencies it is time to get an upgrade - either from Spectrum or buying her own modem + router.
Some ways you can help improve signal strength and coverage is to put the router up as high as possible, and preferably in the center of the house. If you notice that signal strength is particularly bad in some rooms (due to distance, for instance) consider a wifi extender. These are devices you simply plug into a wall outlet and they rebroadcast the wifi signal(s). Place the extender about halfway between the router and the area with bad reception. Again, place the extender as high up, such as on top of a book shelf, if possible. Extenders are about $35-50 - far cheaper than a router.
If the router supports both the 2.4 and 5.4 frequencies make sure both are turned on, and configured. I usually give each frequency its own network name (SSID) with the faster frequency having a "5" in it so I can tell which is the faster one. If your devices support it, be sure to use the faster frequency.
Do not use the default network name (SSID) or password. Everyone can use those, and you don't want your neighbors using your sister's network for free. Choose new, unique SSIDs and passwords. Write them down on a piece of paper and tape it to the router so you can find it. If you want further security, log into the router and turn off SSID broadcast, and turn on MAC Address Access List. The MAC address is a unique identifier every device in the world has. This way you can insure that ONLY the devices belonging to your sister (and you, etc.) are allowed on her network. Unfortunately this can be a bit inconvenient if she buys a new device (new phone) or has friends over, as she'll have to log into the router and add these new MAC addresses to the list.
Internet speed refers to the upload and download speed from the modem to "the internet" (usually a special server in your ISP.) You will never see the full up/down speeds. For instance if your sister is paying for a 5mbps upload, 25mbps download, when you do the SpeedTest, you'll probably see about 4.5mbps up, and 23mbps down. If you aren't seeing ~90% of the speed she's paying for then there's a problem somewhere...
The first thing to do is take a laptop and connect it to the router with an ethernet cable. Turn off the laptop's wifi, and try the speed test once again. If this still gives unexpected slow speeds then it's time to call the ISP and complain. However if the speeds are reasonably close to what she's paying for, then the problem is with the wifi signal. See above for possible solutions.
If, however, everything is working find - strong signal, good speed test results - and things are still slow, this can be caused by there simply being too many devices using the network or using too much bandwidth at once. Streaming a movie from Netflix that is in HD, for instance, can use between 8mbps to 12mbps. Online gaming doesn't use much bandwidth but it needs low latencies (low ping time.) So if your sister has a 25mbps connection and 2 people are watching movies, that's pretty much all of her internet speed right there. The only solution here is....get a faster connection. Either that, or go back to DVDs. :-)
Hopefully something in here helped solve the problem.