Cry when baby cries

My sister and 2 cousins gave birth on this year so there are 3 baby's in the family, two of them are boys and one baby girl. I am very close to my cousins so Every time that I can, I apene time with the babies, specially baby w/ baby girl . She is almost 3 months and is usually  very whiny and cries a lot but NEVER with like "real tears" in her eyes and pouting, sad face and all.. But today she cried, like REALLY cried,  just when I was holding her and feeding her and when I saw this sad, pouting, crying little cutie baby face, out of nowhere I burst into tears too! And I couldn't stop crying!! It broke my heart.. Is this normal?? Or am I ridiculously hormonal...? I repeat it's not my baby, it's my cousins baby!