Can you compare the genetic proximity between two languages? According to this site, you can.
Here are some results (closer is lower)
Croatian-Serbian: 2.8
Afrikaans-Dutch: 2.9
Russian-Ukrainian: 3.4
Danish-Norwegian(Bokmal): 3.7
Hindi-Urdu: 4.3
Dutch-Flemish: 5.6
Czech-Slovak is: 5.7
Gaelic-Irish: 7.3
English-Scots 10.2
Norwegian(Bokmal)-Swedish: 13.9
Danish-Swedish: 15.7
Italian-Spanish: 16.1
Dutch-German: 18.7
Portuguese-Spanish: 19.4
English-Frisian: 25.2