yikes.
you see my age above. i have 2 younger brothers: 21 & 17. me? myself? i see you dealing w/at least 2 separate issues here...
while i do agree w/the "claim your wife" point.. i don't recommend you sit your wife down and state the "too much time w/them, and this and that is not right for a married woman, cuz you are married to me" approach. if this were not a "family" member, then yes. say those things. but we are talking about a step-brother. there is a more delicate way to handle this.
the overall advice is: and i pray you haven't already done this: but DON'T ACCUSE YOUR WIFE OF HAVING AN AFFAIR-EMOTIONAL OR OTHERWISE-W/HER BROTHER-STEP OR OTHERWISE. DON'T TELL YOUR WIFE THAT HER STEP-BROTHER IS FLIRTING. instead, do the following, (in my humble-(i'm pretty arrogant in my posts most of the time, so i am sorry about that...) opinion, please do the following...
both of my younger brothers went thru the phase of wrestling w/me... and it distracted my friends, and their friends. but they went thru it. and while they did wrestle w/me, i had to teach them some boundaries. it was my job. cuz i was the older sister. and it was my job to teach my dad when tickling one day became inappropriate for me. my dad was huffier about it than my brothers were...because they knew. my dad's pride was injured, but he definitely understood. he wanted me to be his little girl. i wasn't anymore. and your wife has the job of teaching her step-brother some boundaries. he's not a little boy anymore. and/or, if he has a wolfish goal, then she has the responsibility of backing him off. but he could be totally immersed, just as she may be, in the "brother/sister" point of view. your wife might accidentally be being suckered. (sorry for the grammar) at the age of 21, it is hard for me to tell, since i don't know him personally, whether he knows what he is doing, and he is intentionally stepping over lines. he came from another family and another background, and because none of us know him, it is highly unwise for any of us to assume that he knows he is doing wrong. and it is unwise for us to assume that your wife knows she is doing wrong.
because she is a sister now, it is easy for her to not look at this 21 year old man as that. she may not have seen a man. an adult man. and it is a huge possibility that she didn't see and doesn't see that she was not doing right by her marriage. what she may have seen was a brother. and being up all night, listening and talking to "her brother" made it easy for her to feel pleased that someone listens to her, takes her advice as a woman, and wants her viewpoint. her step-brother met this need in her. and when he did, she realized that she is not regularly getting this need met. and that she didn't feel like her needs and wants and desires were taken into consideration when you moved. she didn't feel taken care of, nor protected. so she got angry at you. you are her best friend. she yearns and needs the above. and this step-brother put her in touch w/her anger and hurt of the above: i bet she hasn't been able to make many friends at your new location, has she? and do you listen to her much? does she shut down /w you in the past 3 years?? now, she may or may not have consciously realized that her talk w/her step-brother triggered her hurt.
she may look at this guy and see "brother". she will be furious w/you, and it may create a huge huge rift-bigger than already exists-if you impose your "i am the husband" role on her...
but you can say: "i am sad that my decision to move hurt you. i am sad that we are not communicating properly. i want us to get to the point where you forgive me, and how can i help you adjust to this area better? i am sorry you didn't feel like i was taking proper care of you. i am sorry you didn't feel heard. i am sorry you didn't feel like i was taking your advice, feelings, wants, needs into consideration." don't get defensive or argue w/her, if she tells you her anger at moving you again.. just be humble. have an even voice. tell her "i am sorry" numerous times.. let her have her "meltdown." listen to everything she says. don't react. all women have meltdowns. if she has not had one, then she is overdue. sorry. (rueful laugh here)
and in ANOTHER conversation, that has nothing to do w/the above one, find a reason to casually bring up how men think and work!! when their hormones take over, and they are not boys anymore. give her male education, and let her know at what age you realized you had to stop tickling girls unless you were flirting, and let her know when other males learned the same thing, etc etc.. do you see what i am writing?? my husband frequently does that w/me--especially when it comes to educating me about my sons. and i do it to my husband when it comes to the daughters.
again, i say: just educate her about how guys think! we females are totally clueless! and make sure she knows that in your talks w/men, it's not just you, but all guys that think, walk, talk, like this. and make sure she knows that in some things, there are no "exceptional" men.. lol! when she gets the picture, prayerfully she will, she will back off from this guy. she will watch her signals to her brother..
well, i know all this sounds easier read, then done. i am sorry about that. i wish it wasn't like that. but i really have the faith that you and your wife will get this all resolved..
i agree w/the posters. some of this is wrong.. but i don't think it is an affair, or emotional affair..not at this point. i really am pleased w/how descriptive you have been.. thanks.
i will pray for you guys. good luck!!