Birthdays are an individual thing and should be left that way. Siblings are different and thats part of life and should not be treated as if they are the same person.
How fast does the year go when you are living and going about your business. Among my circle of contacts we have already celebrated three birtdays in which have all been on different dates and the people celebrating are as individual as their birthdays are.
Which brings me to this, the latest member of the family side was a child, in which the parents thought that it would be best at this age to give both of them a present instead of teaching them that today is his birthday and when it is yours, we will celebrate yours and give you all of the attention.
Now at first I was going to do the same thing by buying both children a present to keep both happy, but then it occured to me that, the next birthday was happening at the end of the year and niether children will learn anything. So I behaved as i believe and that it that birthdays are and will always be an individual thing- and attention should be left that way-no matter how close together the siblings are- especially within a family- the parents are only asking for trouble- especially if thier children remember what happened