That's not good of you. When you see someone cry, it's tactless to laugh at it. Really it feels wery awkward without such comments, but if you have, as we say here, "eyes on wet spot", you just can't help it, and suffering for characters, you must suffer additionally from your own embarassment. Really you must spare your mom, you're strong enough to keep such words. I always try to hide my tears in such cases. It's kinda neurotic feeling, uncontrollable.