They invited you to dinner on Good Friday possibly to test you. 

It's also a day of fasting.  Why are you concerned about upsetting the relatives..............they
don''t appear to have any respect for your beliefs and obviously are not concerned with upsetting you.
Perhaps you could be unavailable by being in church that day.  

They invited you to dinner on Good Friday possibly to test you.

No, that's probably not the case. They may be just didn't know or no longer practice the Faith. For many people, Catholicism ends at their Confirmation ceremony.

I agree with the poster above who said: they have no respect for you or your religion; why concern yourself with what they think? Who cares if they are mad at you for not coming? It sounds like if they got mad and stopped speaking to you, they would be doing you a favor!

Shake the dust off your feet and live your life as a good Catholic who actually practices his Faith.

I totally agree with Stubborn.  These people are testing you!  I have a friend.  Her only child/son married a Jєω.  They have twins sons.  When my catholic friend goes for a meal, daughter-in-law tells her to say her own meal prayers in another room.  The sign of the cross is not welcomed!

You speak up, and say the truth!

Last week, owe had watch tower visitors.  First at our son-in-laws home just 15 min drive from us.  My husband met them, women, and he was handed a pamphlet.  come to our meeting, she said.  My husband answers, "Oh, haven't you heard, that there is no salvation outside the catholic church?"  She grabs the pamphlet and walks off.    Then the next day, at our home, no kidding, there were these same women!  My husband answered the door, "Oh, I say you yesterday!"  she said, we were not here yesterday.  Husband:  That is right, I saw you in El Mirage and I told you that there is no salvation outside the catholic church.  She left.

And no kidding, watchtower was sitting on our front bench, 8 years ago, Easter Sunday at 11 am waiting for our return from Mass!

So, put some backbone to your backbone and speak up.  This is your chance and they are testing and you let them know, you are in the Catholic Church and you will live and die as a Catholic!

Songbird, that reminds me of a story told by a now-deceased trad (God rest his soul).  He answered his front door one day, only to see these Jehovah Witnesses.  Blessing himself he said, "In the Name of the Father, and of the Son, and of the Holy Ghost."  Then he shut the door on them.  They never came back.

I remember that the Jehovah's Witnesses came to my house one day. I explained that I was a Catholic and I didn't read non Catholic literature. Somewhere along the line they asked for money. I said that I did not have any money to give go them. I never saw them again.