From where did Bani learn this?

From where did Bani learn to say Gajab? After that 7 years leap, Thapki said B for beti, b for Bani. Does that mean she taught Bani to say this?

may be she got it naturally like bihaan because of genes

Even with genes playing their part...Thapki had a great role in making Bani chota Bihaan...coz if she wanted, she could have stopped Bani from saying/doing all those Bihaan-ish things...but instead Thapki was happy to have her Bihaan near her in the form of Bani

Bani had adopted all of this from Bihaan genetically. That was one more proof that she is Bihaan's daughter