You can D/L a hack from Simlogical,then enter the mothers new family and move the twins in.
You'll have to use a teleporter hack.
I'll try to give you a link.
Here it is:
http://www.simlogical.com/sl/Sims2Downloads/Sims2_Teleporters/TeleporterShrubPlus.zipIt's a schrub,called Teleporter Plus,you'll find it in the garden section.
Click on it when in the mothers apartment,click on the family name,teleport them there one by one then chose Move in...
and move them in.