hmm, first thing is that he's going to become older than her are faster, this is an obvious thing but to put it into perspective; a man needs a woman less as he ages, attraction to one's spouse is essential at least from the start, it's an issue to consider at least.
also there might be a generational gap, so they will have to work on that bit too. what do they share? what can they easily talk about? does she want/need to work now/later? he will retire a bit earlier than her, what about the kids then?
the younger sister will also
might not have someone to generally play around with that much, I mean be immature with, younger couples grow up together, and while marriage requires maturity the age difference might factor in more quickly. so the bro will have to be more considerate of that, again it's a 'might', the prophet was much older than Aishah yet asked her about her dolls, raced with her etc. not that a 21 old would have that many dolls ^, but you get the idea.
in the end, she needs to keep in mind the reasons she wishes to marry-anyone- is it love, money, security etc? and he should too. all marriages are difficult, not that I'm married myself :/, so there will always be things to deal with.
to keep a marriage ongoing both need to be able to communicate well, and personally to keep in mind the reasons they married in the first place, the good moments and all that, this is general advice, as remember, marriage is a continuous process, you don't marry and that's just it.
but of course it's not so bad either, well anyway, long story short she should accept if she is ok with him and he with her. all the best to him and her too
