I strongly agree with Moshe Rose (I am shocked).
A Pocket Watch is a Minhag Yisroel, brought down in Ta’amei HaMinhagim, to represent the Shalsheles of Klal Yisroel (continuation of the Jewish people), which is what the young couple is acting on by getting married. Why anyone would want to change such a beautiful minhag is beyond me.
Besides for that, there is no Chiyuv to buy anything. It is “nice”, but so is not nagging your SIL.