Aw fooey on the sister anyway!

If she wants it she'll do it...or not.
You are two different people. Consider her like all the other customers the coach has...none of them affect who you are and what you can do.
Life is full of naysayers. We have to learn how to work around them and ignore them to get what we want. If you crumble so early, you'll never know what you are truly capable of doing. Once you are through these early steps, they will never be able to crumble you again, for you will have seen what's on the other side of them and their negativity!
GO FOR IT!