@Bl08791,
I think that it's natural, you love your sister and want her to be safe, happy.
But, you are so young with a future ahead of you, how are you going to provide for her, as she matures, who will she turn to for "girl" advice. Speak to the other couple that seem to want to love her and bring her into their lives and express your desire to be there a lot for her, if they were to do so and ascertain if they have no problems with that at all.