Andrea's blood has never been tested, AFAIK. Is it going to turn out that the red haired woman is their birth mother? Perhaps it'll turn out that Dee's mum couldn't cope with 2 babies so gave one away. Then she got together with Dee's "dad", had another baby with him & they just decided to raise Dee as his. Their mum was too ashamed to admit she gave her baby away so never told him but tracked Andrea down to check she was ok.
Or perhaps both babies were adopted & when the mother had another child they asked the adoptive parents if they wanted to adopt her too, hence the bio-sister.
I can't imagine Andrea's going to get v far with impersonating Dee this time. I'm picturing a scenario where Andrea heads back to Toadie's doing her very best Dee impression. Heather arrives just after she's left & hears Dee shouting for help. Thinking she's Andrea, she helps her up & takes her back to Toadie's because she says she "has to stop her." Heather assumes she's Andrea & needs to stop Dee. They arrive just as Andrea is telling Toadie that this mad woman who looked just like her attacked her after talking about him so she pushed her over a cliff in self defence & came to check he was ok. Andrea uses the fact that Dee is with Heather to try & convince Toadie that she's the real Dee. Mark sees Heather arriving & comes to arrest her. It's apparent that Andrea has attacked Dee but it's all on Toadie to figure out which of the twins should be done for attempted murder...
Real Dee will convince him with details only they knew & once Heather & Andrea have been removed, after Heather has explained the separated twins thing, Dee & Toadie sit down & talk about the accident & why she hadn't contacted him. She'll explain she was too traumatised to face her previous life & has moved on. She's happy & she hopes he can be too one day. [Insert fab Toadie crying here.] They'll say their goodbyes. She'll bump into Susan on the way out & gently say it's good to see her & she'll let Toadie explain before going to her cab, taking one last look around & leaving Ramsey Street forever.
It won't be Sonya's death good but it'll produce a few sniffles at least.