One filly, and two mothers

One fateful day in Equestria, the royal palace was all a buzz, considering their normal morning tasks to do to start the day. Luna lower'd the moon so her sisters sun could peek through the land. Then there was breakfast, which was always quiet, well, mostly anyway. Then at breakfast, they read over there tasks for that day, and so on. So really, it was just a normal day for the princesses, until...

They heard a loud thumping on the door. Celestia was about to get up from her seat, when one of the royal guards got the door. "Your highnesses, there are two ponies at the door that want you two, to help them." Celestia got up from her seat. "Are you coming Luna?" "Yes in a minute." Luna said with a mouthful of pancakes.

A few minutes later, they were in the throne room with the two ponies. "What do you want?" Luna said questioning them. "Luna! Don't be so harsh on them!" Celestia said, basicly scream/whispering. "What did I do?" Luna said confused. "You, you. Ugh, nevermind." Celestia said, sighing. "Uh, your highnesses, you have ponies still here waiting for you." The guard said. "Oh, uh, right. Alright, you may start now." Celestia said, just a little ashamed, that there were still ponies waiting on her. "W, we, ju, just, wa, want, you, tttt, to help, our, our little fi, filly. The mother said crying so hard that she could had filled up a lake. "Your filly? What help are we to your filly?" Celestia said really confused why they came here. "W, well, I, th th, thought, that you two, could s s s, save my dying d d d d, daughter." The mother said so sad it made you start crying too. "Shouldn't you just go to the hospital? To check her out?" Luna said. "Please help us, I already tried the hospital and they were no help. So please help me!" The mother said screaming. "Well we might be able to do so but_  yes go on, the mother said blurting out. "But, as I was saying, on one condition. One, just one." Luna said also kind of ticked off at her being cut off. "Please anything!" The mother said willingly. "Anything?" Luna said."A-N-Y-T-H-I-N-G please!" The mother said spelling out the word. "Fine then, sister, do you agree with this?" Luna siad. "Yes, I do but, on what condition?" Celestia said. "Making her a changling." Luna said quietly so the mother couldn't hear. "What?!" Celestia said shocked and confused at what her sister was planing to do, to that poor filly. "I think its the only way." Luna said. "Why?" "Because, my only other option I have is for us to take care of her, without her mother." Luna said. Celestia sighed. "Well, do you, have any bright ideas?" Luna said questioning her sister. Celestia sighed again. "No, I guess not." Thought so." Luna said to her sister, not that happy about what she was about to do either. "Here it goes." Celestia said, closing her eyes about what was to come of this plan. Luna's horn glowed brightly and...

Then the filly opened her eyes, crying, even more than her mother did. Alive, living...

and a changeling.