Is the robe listed on her registry?
If not, if I were you, I wouldn’t get the robe - unless you just KNOW it’s the kind of thing your future sister-in-law would like.
For a bridal shower, I usually purchase a household related item/items off the registry - I’ll spend more for family.
What I purchase depends on what’s on the registry. Sometimes, I just go w/ one larger item. Other times, like other posters have suggested, I may get a few different but related things & put together a themed gift basket. Depending on what the items are, I may add a few things that may not necessarily be on the registry but are related to the overall theme of the other items.
For the wedding itself, in our area, it’s customary for the couple to receive both gifts &/or monetary gifts at the wedding. In other words, at weddings, you’ll usually find a gift table w/ wrapped gifts for the couple, but the couple will also receive cards w/ checks & cash included.
So, again, for family, we spend more on our wedding gift. In some instances, we’ve just given a card w/ cash at the wedding. But, for others, we’ve sometimes purchased something off the registry for the wedding gift as well - usually an item that is more lasting or significant than a regular household item like bath towels... maybe a piece of their china or something similar. And then we also include a monetary gift. If we’re giving both a wrapped gift off the registry & a monetary gift, the amount of the monetary gift is not as much as it would be if we were ONLY giving a monetary gift.
I usually only do personalized items if we know the bride or groom really well. For instance, for one wedding, in which I knew the bride really well & knew the kind of things she liked, I had a personal print w/ their names & wedding date made & framed, & we gave that for our wedding gift along w/ monetary gift in the card.