Brother this a cultural question. What Islam teaches us is you should not be alone with a non-Mahram and you should keep good relationship with kin. It is also better to separate men and women that are non-Mahram in family gatherings, but this too is cultural mainly.
I would really be upset at my brother if he hid away due to protection of his wife or fear I would somehow try to corrupt her somehow. That thought never even crossed my mind, subhanallah. Your brother needs to improve his character, confidence, and mature a bit inshallah if he is saying such things. It is really a strange thought and even stranger that he has expressed this to you. Maintaining good relationship with you after marriage is very important, he should not marry a particular person if it will hurt family ties or pull him away from your family. Marriage should only strengthen and unite families, not cause division, fear, jealously, etc. This is of course as long as your family is not going against an Islamic principle or right of yours, you don't have to listen to them then but do what you think is best inshallah for your deen. Please be reasonable and sensible. Just because you see his wife or give her Salam does not mean it will lead to evil, rather it is common courtesy you should give out of respect to your sister and wish her the best. If you have evil thoughts then it is waswas of sheytan, just ignore it like you do all the time.
When you two are married you will see that most of these concerns are silly inshallah and you will mature up. When you are married you will have bigger problems then if your wife was seen or talked to by your brother. I think jealousy of your partner is good but taken to extreme, like this, will cause more harm then good as it is not needed. So get married first, then worry about this later inshallah. I don't think you have anything to fear. Just try to be kind and respectful as always and you will get along fine with your brother and he will become more comfortable around you with his wife/family in time inshallah.
I also dislike when a brother says, "Oh I am a father now" even though I been in close relations with the brother for many years and through the 9 months of pregnancy. Or worse, "I am the farther of 4 now" when I did not even know he was married or had kids. lol I pretty much write off these people as being my close friends or trustable at that point as they cannot even share to me this good news of a baby on the way to get my dua for them and they are so secretive. Nazaar (evil eye) is one thing, but these brothers have psychological problems in my opinion and I don't like such cold people that are afraid to talk about such simple matters. I don't think they can be trusted in a lot of ways and since they don't talk, they are really just there to use you and not be a good friend or brother to give you sound advice and wish you well (they just fear you). It is quite a hurtful thing to me, this is just my personal opinion and one way I use to judge friendships and people on. I see such brothers as quite backward, lacking good cultural norms and good character. I wish the brother all the best still and understand that everyone is different, but I just don't find much in common with such people as I am more of a transparent person.
And just the opposite if a brother tells me "I am getting married" or "my wife is pregnant" I feel such joy and get close to this brother as I can see he really loves me and wants to share this good news with me. I also make lots of dua and say mashallah to save him from Nazaar.
