Chelsea,
There is difference in perspective that is in play. While those who are important to us tend to think of our transition as a loss, a disappointment, even, a death, of sorts, we look on it as more of an awakening, even, a birth. You need to recognize that your mother is in a mourning. Hard to sympathize with when you are in a celebratory mood.
Allow mom to mourn. But try, through your sis if necessary, to encourage mom to see this in the same positive light you do.
Hang in there,
Stevi