surname, marriage

If it is not because of the law of the land, you do not have to change your name, in keeping with the Islamic traditions. None of the women (RA)of RasoolAllah (SAW) and his (SAW) companions (RA) changed their names on their wedlock and continued with their fathers names. (Quran does state a case of an adopted son who must continue to be called after his natural father's name). WE must follow Islamic traditions. NOT doing that and instead following the ways of kufar despite having a choice could be a sin.

It is not sinful, but it is not forced either.

I have no proof but If her last name is the same as yours no it's not sinful but It's not traditional or part of Islam to change the women's last name to the mans.