Check with your surgeon. In NYC the view/data is that sleeve may cause more problems than prevent if used BEFORE/absent a concrete lymphedema diagnosis. Your surgeon will have opinion/protocol for her/his practice and will be able to further explain her/his thinking. There are exercises that surgeon or staff will give your wife that may be especially useful when flying.
Other thoughts:
Radiation - you did not mention but with lumpectomy radiation usually starts 6-ish weeks post surgery - a bit longer for mastectomy - and is M-F for up to 6.5 weeks. Since you can't subject recently radiated skin to sun/sand/chlorine might be best to fit holiday btw surgery and radiation.
Chlorine/Ocean/Immersion: Some surgeons restrict swimming for longer than others out of fear of contamination. Also keep all scars/incisions covered from sun if having scars fade is important. Also - remember she will remain photo-sensitive for a year-ish after chemo so lather on the SPF and stay in the shade where possible.
Expander - if your wife does opt for mastectomy and reconstruction she will have an expander in when you go on holiday. At the 6-week mark it will probably not be 'full' so not the same size as the other side. Some women won't care but some would be very self-conscious about this in a swimsuit. Your wife can think about that and let you know where she falls on the issue. In addition the expander can be a little uncomfortable. Anecdotally it seems like women with a slim build find this to be the case more often than others. Your wife may want to reach out and see if anyone in her BC treatment cohort knows anyone with personal experience flying with an expander.
Activity - do not let your wife even pull a suitcase, never mind lift, on the BC side. No bags, no weight, no stress on that side - especially after a mastectomy. Also - consider that some people do have pain/range of motion issues after mastectomy. Surgeon/PT will let you know if swimming is advised if there are range of motion issue.
You seem like a very supportive husband. I recently hurdled BC and having a partner who could roll with the punches and look forward to a fun future helped. Free advice (not applicable if you have already weathered AC portion of chemo): your wife should be drinking plenty of water now - but if she is doing the AC at the end she should start drinking water 'like its her job' - about 3 liters a day if possible - the week before and then all the way through till 2 weeks after last infusion. Again - run by your Dr - but this advice I got from someone who had been through it helped me stave off the worst of the side effects. Also - be proactive with anti-nausea meds - no prize for suffering through without them and easier to pre-treat than make go away once it starts.
Wishing you a great outcome and many, many years of fun travels.
amsNYC