I received about a dozen Kiddush cups as wedding gifts. B"H, I have used almost all of them for some reason!
Offhand, I see no problem with giving a couple a Kiddush cup for their wedding, regardless of whether family members do likewise. (Interestingly, I don't recall any family member giving me a Kiddush cup.) IMO, you can't have too many Kiddush cups. Many people I know reserve certain cups for Yom Tov and additional ones for Pesach. (I've heard of some people that use a different cup for each of the 4 cups during the Seder!) If the wedding couple invites their own large family and / or many Shabbat guests, they will probably want every male guest to have his own cup. So, you can see that there are many occasions where even a dozen cups (not necessarily all from you ;-) still may not suffice!
Most people use silver cups. They are the most valuable, increase in value over time (usually), are easy to clean, lightweight, durable and non-breakable (though, they can get dented if abused.) Some people like clay cups / sets, and many beautiful ones are available. You may want to find out if the couple has a preference. However, as stated. clay / pottery ones stain and chip far more easily than silver does.
While I know that silver is expensive, I strongly advise against silver plate, though I know it's now cheap and "trendy". You pay a lot more money than it's value, and it has a tendency to stain and become dented because it's a bit brittle.
Also beware that some of the plated as well as full silver ones, esp. the ones with stems tend to be "weighted" on the bottom with wax! Yes, wax! Often dealers don't tell you this, and make you think you are buying weightier silver, and you paid silver weight for wax. Go to a reputable dealer or know what you're buying!
Another factor - Many people place a silver plate under their cup for Kiddush. Years ago, the plate was included in the price of the set. This is a rarity, now. While you do not need to buy a plate, this could be a factor in your total gift cost. My opinion if you're buying both - get a fancier cup and a less fancy plate if cost is a huge factor. You also, of course, don't have to buy both from the same place at the same time!
I don't advise buying online, as it's hard to tell what you're getting. But, if you do search online, consider eBAY. I've bought some nice antique silver cups on eBAY and have been quite pleased. Just research well and ask the buyer plenty of questions until your satisfied, and do not buy anything on eBAY from a foreign country that is not bound by U.S. legal buying laws (such as China). VERY risky!