In most cases their functions are the same.The only diffrence is that when you use "whether",you can use the pattern of "whether ...or not".However,when you use "if",you cannot use that pattern.
For examples:
I don't know whether she will come.
or:
I don't know whether she will come or not.
I don't know if she will come.
But,you cannot say:
I don't know if she will come or not.
Besides,sometimes it is not bad to use a different word when too many "if's" are in a sentence.
For an example:
If you are not sure about her decission,why don't you ask her whether she will come to your birthday party or not?
Compare that with the following sentence:
If you are not sure about her decission,why don't you ask her if she will come to your birthday party?