It sounds to me, too, like a hormonal thing, but someone could perhaps point her to some form of therapy or a neurological checkup.
You say she doesn't go out to work - is she perhaps a little frustrated with herself, despite doing a good job running a home, family and pets? Your SiL shouldn't stick his head in the sand, either - this could be important, and it could get worse.
It's certainly nothing to do with you or the dog whining in the night. Throwing the present back at her husband is irrational behaviour and she may do this at any time and harm her kids or her pets.
Sorry to sound so negative. It's just that it would be sad to see a family who is obviously otherwise so normal and peaceful go to pieces because of something that could be rectified.
Also, following on from Greyduster's comments - can you, as her mother, perhaps relate to this behaviour? I know that I can totally sympathise with my DS2's apparent idiosyncrasies, and I am learning about myself in the process!
Please try and find a way of helping her, for your own peace of mind. I hope I haven't worried you - these are just my thoughts, without knowing the whole story, of course, as are the previous posts the thoughts of all who have read your story.