You have no need to try to give that gerbil away; they are social animals who live in groups and take up little space and require little food. Watching them interact together is always enjoyable.
Leave the sister in. Leave the mother alone, and give her more protein sources, such as bits of bread soaked in evaporated milk. Don't touch the nest or babies. Give them peace and quiet.
I think in 5-6 weeks they will have to be sexed and separated by gender.
Keeping pets that you have raised since birth will result in VERY tame animals, and same gender groups that are especially close. A group of three or four will be very enjoyable to watch, especially as each one's individual personality develops!
I don't know how many babies you have, but gerbil litters aren't that big. No reason not to keep them all, and many reasons TO keep them, as I said.
I have 23 gerbils here, btw.