Your sister is right. You need to start sealing.
How long is your hair? Sometimes the scalp's sebum production just isn't enough to lubricate and protect the hair shaft and you have to help it out. Usually the longer your hair gets (or the curlier your hair is), the more oil your hair needs. Also, another point is you may not be using enough conditioner. And if you want to keep using shampoo, make sure to condition more days per week/month than you shampoo. On the off days, just rinse your hair with water and use a ton of conditioner, focusing on the ends. You can apply the oil to wet hair after your leave-in conditioner or moisturizer/styler.
In addition, if your hair touches your shoulders, the ends (that are probably at least 2 yrs old at that point) are getting more wear and tear on them, causing the ends to begin splitting. Oiling your ends on a regular basis can stave the issue, but make sure that when getting trims you are getting the strands cut above where the hair shaft if splitting (at least 1/4" above). Otherwise it will continue to split and you'll continue to spin your wheels. Start out with a lightweight oil (like apricot kernal or sweet almond oil) and apply a little to the last 2-3 inches of your hair and you should see a difference pretty quickly.
To me though, besides issues in your regimen (hair not getting enough moisture) it seems like the trims you have gotten were insufficient (not enough was cut to prevent the splitting from recurring). Make an effort to learn your hair's properties (texture, porosity, and density) to make the most out the time you do have to care for your hair.
4a/3c, fine strands, low porosity, medium density
Last relaxer: Jan 2010 - BC'd: 2/27/11