your mother had 50 per cent share in the flat
2) on her demise her 50 per cent share will devolve on you and your father
3)your father can execute relinquishment or gift deed for his one fourth share in flat
4) you should then transfer mother share in flat in your name
5) your aunt can execute will bequeathing her 50 share in flat in your name
6) on basis of will apply for mutation of aunt 50 per share of flat in your name
7) probate of will is not mandatory in UP
8)you will save the stamp duty costs on transfer of flat in your name