Second cousin marriage

she is your father's immediate sister's daughter  then as per Hindu customs it is allowable 

Sir his mother is my father cousin. So his daughter is my second cousin and cross cousin

No, you can't marry .It comes under prohibited degree.

The girl is the daughter of your father's sister(cousin).  In some Hindu families, the girl's mother is your aunty(BUVA) and so it is allowable marriage.

Sir his mother is my father cousin. So his daughter is my second cousin and cross cousin

respected sir According to Hindu Marriage Act, persons are said to be under the degree of prohibited relationships if-

1.  If one is a lineal ascendant of the other

2. if one is the wife or husband of the lineal descendant or ascendant of the other

3. if one is the wife of the brother or of the father's or mother's brother or grandfather's or grandmother's brother of the other

4. If the two are brother and sister, uncle and niece, aunt and nephew, two brothers or two sisters, children of brothers and sisters.

5. they are not Sapindas of each other (unless the custom allows them to do so) hence a person cannot marry up to third generation in the line of ascent through the mother and fifth in the line of ascent through the father.

regards
 

Hello,

Your realtionship is within prohibited degree. You cannot marry your father's cousin sister's daughter. 

The proposed relationship comes within prohibited degree. Marriage impermissible unless there is a custom to the contrary in your community.

However, such prohibition is restricted to the parties in marriage, not the world at large.