I can almost guarantee that you will not be treated badly even in the slightest should you decide to convert to Catholicism. The Catholic Church is an ancient organization, and it has tons of experience with people converting to the Faith. It is one religion. It does have different flavors, so to speak, but the doctrine is exactly the same in all of those flavors. Protestantism has something like 30,000 different denominations, the differences between denominations sometimes coming down to such petty things as what instrument they play for church service. Naturally, you’re going to get about 30,000 different kinds of reactions from Protestants when you tell them about your family situation. Some reactions will be negative, some not so much. The Catholic Church is made up of people, and so there will still be a range of reactions, but I can almost assure you that no one will react negatively. I am a convert, and very few other people in my family are Catholic. People at my church generally either don’t really have a reaction when I tell them that, or if they do, it’s perhaps slight pity or something like that. Actually, it’s not rare to meet even a priest every once in a while whose family is not Catholic. There are even lots of saints who were converts.
No one in the Catholic Church is going to freak out when you tell them that your family are non-Christians. It’s really a non-issue. There are tons of Catholic converts whose families are in the same situation. The Catholic Church, as I said, is an ancient organization. It has a lot of wisdom in a huge spectrum of issues that Protestant communities simply do not have.
Like I said, non-issue.