They want to conceal their marriage

I am converting to Islam, however I find it difficult to tell my family..I will do so in time however is it necessary to announce it officially.
I wish to also become a wife to a man (who already has a wife and children) for islamic reasons. The difficult part is also telling his family
Is it necessary to annouce our marriage? As we do not wish to hurt anyone.
I trust you will be able to guide me as will Allah.

Praise be to Allaah.

You do not have to announce your Islam, whether officially or to your family, and it is permissible to keep it a secret if there is some fear of harm. As for marriage, however, the sunnah is to announce it, because the Prophet (peace and blessings of Allaah be upon him) said: “Announce marriages.” (Reported by Imaam Ahmad in Musnad ‘Abd-Allaah ibn al-Zubayr; classed as hasan in Saheeh al-Jaami’, 1072). If a marriage is announced in one city or location, it need not be announced in another, but we do not advise you to conceal the news of your marriage, especially since such news usually becomes known eventually. This news is likely to be a big shock to the first wife, so announcing the marriage is in the interests of all concerned, as it establishes fair and just treatment for both wives. And Allaah is the Source of strength.