As per the provisions of the Hindu Marriage Act, 1955 (HMA), two people are said to beb sapindas of eachother if one is a lineal ascendant of the other or within the limits of sapinda ( 3rd Gen in the line of ascent through the mother & 5th Gen in the line of ascent through the father) or if a common lineal ascendant is within the category of sapinda to either of the parties proposing to get married.
Mother's unce's sisteris mother's auntand also falls within 3 generations of sapinda calculation from mother's side andhence, prohibited. Such a marriage would be void. Now, a question arises as to the admissibility of a step mother in the sapinda calculation. The Privy Council in the case of Lallubhai Bapubhai v. Cassibai I.L.R. 5 Bom. 110 held that upon marriage, a husband and wife become sapinda relations to each other and therefore, the step-mother becomes the Gotraja Sapinda of the step-son.
Based on the above, your marriage with mother's uncle's sister will be void.
The Hon'ble Delhi High Court, while deciding a case on the subject issue held that marriage between two Hindu cousins after converting to Christianity was valid. However, the Supreme Court has also ruled that religious conversion for material gains is invalid.
In light of the conflicting decisions above, it is advisable to seek professional legal help and proceed with due caution.