Well, if it makes it any easier, if they know you are in RCIA, they may be expecting this (granted, a lot of folks who attend RCIA do it just as an inquiry, but still…)
You need to tell them about your entering the Church. For one thing, it’s the right thing to do (Jesus said that whoever denies Him before men, He will deny before the Father…don’t be afraid of professing your Catholic faith!) Also, they’re going to find out about it eventually, one way or another, and if they find out that you’ve been hiding it from them, that would probably go over worse than if you simply be frank and tell them.
I would suggest avoiding getting into a debate with them when you tell them. I imagine that emotions would be running high, and a debate could easily get out of control, and, as you said, they don’t seem to be open to discussing things. There will be plenty of other occasions to witness the Catholic faith to them and explain your reasons/convictions for converting. In the long run, if they seem resistant to discussing things, you could always write a letter discussing your reasons for converting (David Currie did that, and it ended up turning into a rather good apologetics book
) People tend to be more receptive to books, recorded talks, and other forms of pre-recorded media that they can’t get into an argument with.
They may see it as turning your back on the family or abandoning your faith, but perhaps it would be helpful to turn that reasoning on its head and, first, thank them for giving you a solid Christian upbringing. Explain that you are doing this out of obedience to that same Christian faith, that you have become intellectually convinced that the Catholic Church is, in fact, the Church that Jesus founded, and that you feel compelled by conscience and obedience to Christ to join it (but, again, don’t let it turn into a heated argument).
I converted to the Catholic Church after leaving the Lutheran Church Missouri Synod. However, I had it pretty easy; my parents were pretty supportive of me (one of them still attends the LCMS, the other hasn’t been a church attendee since his childhood). It also helped because I had stopped being Lutheran 5 1/2 months earlier, and my parents knew that I was looking to convert to either the Catholic Church or the Orthodox Church.
But I wouldn’t bring it up unless religion is talked about and you’re brought into it. Then I’d bring it up and explain why to your family. Luckily for me, I’m in the ELCA. 
Good luck on your journey and God bless. 