Dear Sister Assalam o alekum,
Thanks a lot for your replies, your posts, your time.
Allah ta'ala aap ko apni janab se jaza ata farmaye.
Sister, I do understand that respecting and obeying parents in-law is a moral deed (Ikhlaqi faraz) but main reason behind asking this question was that obeying them is not religious duty or Isamic sharia'at faraz.
After reading this above fatwa, I can make out that it is not religious duty, hence if it not fulfilled, there is no 'gunah' or no such liability by sharia'at.
Lastly, Please check following links I've found while searching net:
http://qa.sunnipath.com/issue_view.asp?HD=7&ID=9058&CATE=3640
http://qa.sunnipath.com/issue_view.asp?HD=1&ID=273&CATE=87
Do comment on this.