tell me something...if you consider your brother and his wife dear and their home as your own family, why do you need her to specially welcome you? in fact, you should yourself have suggested going after the exams were over.
even your SIL could see this as disturbance where you have no regard for her son's studies. also, even if you go there, why does she have to specially welcome you all? if it is also your house, you could offer to help her out and rest for a while and not be formal at all. it is these petty thoughts that create so much unnecessary issues in families.
Read more