I answered this question yesterday. I'm not a Jehovah's Witness yet I am a student of the Bible and a follower of the Christ. For anyone who is interested, here is an example of exactly the attitude that the majority of Jehovah's Witnesses reflect so everyone can see that I was not exaggerating in my original answer.
This Yahoo! user name is "roamiblu" and this person obviously does not know the correct definition of an Apostate. An Apostate is a person who speaks against God and/or Christ and his holy word the Bible. This Yahoo! user "roamiblu" instead of providing an answer of substance, this Jehovah's Witness attacks my character and my anonymity, which is of no consequence, exactly like my original answer described the Pharisaical attitude of the majority of Jehovah's Witnesses reflect, at least many of them. The following comment is Jehovah's Witness member and Yahoo! user "roamiblu"'s comment on my original Yahoo! answer which follows immediately:
"roamiblu" wrote:
"Words of a TRUE apostate, that is you. Anonymous if you are going to say that you are a "True follower of the Christ to the glory of Jehovah God" then you should at the very least be proud of it and display your real name. Stop living in the past. Study the Bible then apply Proverbs 4:18."
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It saddens me to see how unloving some responses have been towards your sincere question, specifically those coming from so-called Christian Jehovah's Witnesses. At the very least, they should be loving and caring in their replies. Instead they behave like the Pharisees, condemning and condescending. There are ways to give someone the Good News of the Christ and they fail miserably in their feeble online attempts.
Many of the festivals that are celebrated today do have origins in pagan customs, no disputing that. Celebrating many of them is not honoring God. But seeing how your brother has changed does not surprise me at all. The Jehovah's Witnesses while they do teach many biblical truths, at the same time, they sprinkle their own man-made teachings. And those man-made teachings and bible interpretations are not open to question whatsoever. If you question or go against their Governing Body arrangement, you will be kicked out or shunned.
The Jehovah's Witnesses have love-bombed your brother and more than likely have told him that ANY opposition or negative comments, specifically coming from relatives and friends, come from only one source; Satan. Therefore, your brother sees you and your relatives like demon-possessed people who are out to get him. So every time you or your relatives bring up ANYTHING related to his new found faith or that portrays the Jehovah's Witnesses in a negative manner, it reinforces the thought that has already been implanted in him.
And while the Jehovah's Witnesses would like to rightfully claim to be the only true-religion on the planet, sadly they cannot. Why? Their Organization or Governing Body has continually prophetically set 'end of days' dates and teachings which have had to be changed over and over and over and over again. And as they would say, they consider it "new light" or "refinements" or "clarifications" or "adjustments". They always point to Proverbs 4:18 in justifying these changes. The context of Proverbs 4:18 does not teach this. Biblical teachings are set in stone, they do not change with time, only human-origin interpretations will change. The fact of the matter is, without Holy Spirit humans will never be able to properly interpret scripture. Which only proves that God has never given this wonderful gift to the Jehovah's Witnesses Governing Body otherwise how else would they be so wrong time after time? The correct teachings they do teach and have gotten right over the years have been a result of cross-referencing different scriptures. It's similar to how a detective would go about solving a crime. While the detective will have no diving help, he does have his training, education, and intuition. More than enough to help him solve the crime if all the evidence is right in front of him.
See, the thing is, the Jehovah's Witness Organization like to claim God's sole Channel of Communication for humans, yet when their interpretations don't come to fruition, they also do not claim infallibility. So they wish to have it both ways. And if they adhere to "what the bible really teaches", they would see that in Deuteronomy 18:22 Jehovah God calls these kind of people/organizations False Prophets.
I can almost say with 100% certainty that your brother does not know the Jehovah's Witness history otherwise, he would see that they are in fact what Jehovah God clearly describes in his word the Bible; a False Prophet.
The very best thing you can do is to patiently provide to him evidence on many of the Jehovah's Witness teachings where they are clearly in error, hence false prophets. Jehovah God deals with each person individually being that we will all be judged individually not collectively.