well if it's not much.. then just let it be. Just like your spouse - be kind and polite, but don't initiate contact.
I suppose If you want to address it with SIL, you can. If you want to just tell her that you are really busy at this time and will give her a ring once things slow down - you can do that too.
I know it's really hard but there is no right or wrong answer here.. only the one you have live with.
M(f): 40 D'ed: 8/12
Show empathy when there's pain. Show grace when warranted. Kindness in the midst of anger. Faith in the face of fear.