Your mother sounds a weeny bit barking. I am sending up a small prayer that I won't be like that if ever one of my children looks like producing offspring, and discusses naming with me.
Sorry, I'm sure your mother is a lovely, lovely person, but Why did she say that to you???? Your sister doesn't even have a partner yet: fate-willing, it may come to pass that she has many, many children, but it is not certain.
Is your sister very highly strung?
If there is more to this, then you can always lie. you can say that your dh chose the name, after a teacher who was particularly influential, or something. Anything.
And the thing is, it's pretty crazy anyway, because many, many people have the experience of looking at their newborn and realising the name they had chosen just doesn't seem to suit anyway.
So, not only might you change your mind, but (should she ever actually be in the position of choosing a name for a child of the apposite gender) your sister may well do, too. And what if her future partner hates that name??
Sad you've been put in a fret about this. But surely the thing to do is to choose what suits, and then deal with the fall-out diplomatically.