You've given a lot of good reasons for why you chose March 2015 as your ideal wedding date and people are ignoring them all to make comments about being worried about your hair or being a bridezilla wanting to steal your sisters thunder.
Is your sister Thor? Because otherwise she has no thunder to steal.
March is a reasonable amount of time away from your sister's wedding, particularly as her wedding is still a secret wedding hardly anybody knows about.
Especially since you were only told about this secret wedding after you and your DP had discussed your options and revealed you were thinking of March.
What would they have said if you'd done the same thing as your sister, and booked the wedding without saying a word? They could hardly make you cancel it.
You are being very generous to your sister. She's already vetoed May onwards as being too close to her wedding, and you've given very good reasons for not wanting to wait until October or later in the year, including TTC, your son's school, your grandparents age, their worries about traveling, your guests jobs and holiday entitlements, and your brother's emigration plans.
If your sister had posted to say that she had booked a secret wedding she wasn't telling anybody about until her DP had done a proper proposal, which he had been promising all year but still not gotten around to, but that in the meantime you had made wedding plans of your own, without knowing hers, and she was upset because your March wedding was now too close to her secret July one and she wanted you to cancel it until 2017, this would be a very different thread.
I doubt anybody would be telling her that she was being perfectly reasonable because you could get married right now, or at Christmas this year without your grandparents there, or at October next year without your grandparents and your brother there, or that you shouldn't have a wedding at all unless it cost to 2p in the registry office and your hair looked crap in all the photos because she had booked her secret wedding first.