Does the marriage contract become void, and is there sin, if the spouses agree to reside on a separate floor in the husband's family home, after the marriage contract stipulated living away from the family, and the spouses had previously lived in a distant area?

If the wife is an adult and of sound mind, she has the right to waive any condition stipulated for her benefit, because she is the owner of that right. So, if she waives it, it is waived, and this does not affect the validity of the marriage contract. If the wife agrees to live with the husband's family, there is no harm in that.