While contemplating marriages , the History of of both boy and Girl are deeply studied.
We have GOTRAM, and Rushi Pravara.
If both girl and boy are of same Gotra , they become brother and sisters because they descend from same Blood (their elders are same ).
Similarly the blood relation is SAPINDA relation.
FIVE generations on father side, SEVEN generation on mother`s side , the proposals are not considered, as per shastra.
Sapinda relationship" with reference to any person extends as far as the third generation(inclusive) in the line of ascent through the mother, and the fifth (inclusive) in the line of ascent through the father, the line being traced upwards in each case from the person concerned, who is to be counted as the first generation;
Also chk Hindu Marriage Act
in my opinion it is within Sapinda relationship
Now if you get married if no one comes to know, nobody is going to object, but when the validity of the marriage would come into question while deciding some other issues on the basis of marriage, this will defntly create problem