y u had to cancle ur husband's birthday party....
was it jst two of u or u invited other people too....
how about you let the kid join you or u guys can go out for something and take the kid with you....
if your husband always talk to u before replying on ur behalf and this was the only time when he hurried up and replied than i think there is no issue......
kabi kabi ho jata hai....
as others said y dont u talk to ur SIL nicely that sometimes its hard to change schedule at the last minute when she only informs ur husband and not her....