However, to reply to someone above who said it doesn't really matter if the name is changed, then I would say thatit is a form of control and our name forms our identity, so its also a subtle attempt at controlling the persons identity. And, even if there are two same names in the family, that does not in my opinion justify changing the d-i-l's name - if it bothered them so much then they shouldn't have gone ahead with the marriage.
I would never ever let anyone change the name given to me by my parents, although sometime I do wish that they had given me a more exotic name!!!
Ditto to that sister. See that's exactly what I think! Just wanted to add that if it is a practice that is followed in their family then I'd take it as an assault.