Gift from relatives is exempt. Father in law is covered under the definition of relative. Hence, no tax liability if she gets gift from her Father in Law.
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2 Jul 2018
Relatives for tax free gifts include:
Your and your spouse’s brothers and sisters
Brothers and sisters of your parents
Your lineal descendants (including spouses)
Lineal descendants (including spouses) of your spouse
3 Jul 2018
I would rather suggest you to not send it as gift rather send it as loan taken to buy the flat,
and any interest paid back to your father can be claimed as deduction from taxable salary of wife.