Crushes

Max has two new friends; twin girls age 1,5 ;-). He loves it when 'the babies' come over.

They look very much a like and for the longest time I did not know which one was who. 

But apparently Max does, he is absolutely in love with one of them, and totally ignores the other. He talks to her in his sweet baby voice, goes to her level and is continuously hugging and kissing her.

Why only her and not her sister?