Two years into my PhD program, and Mom is dying of cancer. Should I tell my advisor about it? I just finished my 2nd year of PhD, and my mom has been dying of cancer since the end of my 1st year. She nearly died in January but thankfully pulled through. She was just diagnosed again a few weeks ago and her prognosis is not good (resistant to chemo and can't get another surgery). 

My cognitive abilities and general state of mind have suffered as a result of this situation. My research is very mathematical and being unable to stay focused for longer than 20 minutes really prevents me from making any progress. My emotional/mental state has also been seriously affected. I managed to pass my qual and aced my classes the past 6 months, but lately things are getting a lot worse.

I am hesitating if I should tell my advisor about my mom, but I feel that I'll just sound like I'm making excuses.

Quitting is NOT an option because I have never thought about it and I always wanted to do research since I was in high school.
My deepest sympathies for what you are going through.

You should certainly tell your advisor. No reasonable person would believe that you are using this situation as an "excuse" for poor performance.

Understanding your situation will help your advisor do his/her job: advising you according to your current needs and circumstances.

Your advisor may be able to direct you to mental health and other support services at your university that can help you through this time. He/she can also make you aware of other options that are available under exceptional circumstances: for example, a short leave of absence if you feel it is necessary, or an option to delay a comprehensive exam, or other similar concessions that can take some of the academic pressure off you while you focus on regaining your mental and emotional health.