Personally, I try to keep it simple and say that it is for (a) modesty, and/or (b) because Islamic law says to do it. You could also say that (c) because it is something you want to do and is important to you. 

The aforementioned statements may also be suitable for you to say, however it is  important to say them in such a way that would not imply that you are criticizing your mother. (For instance, if you say that hijab adds dignity, you would not want to say it in such a way which would imply that your mother has less dignity, because she does not wear hijab.) It is often how we say things that is as important as what we actually say!

However, if they are religious Catholics, mentioning the Virgin Mary might be a helpful way to connect. 

I am sure that in your heart, you know what is best to say for your situation - what comes from the heart reaches the heart. 

Best wishes!