Like someone mentioned, the 85mm 1.8G is a safe bet, but I am gonna politely disagree that it is the absolute king of creamy rendering. To be honest, the 1.8G can still feel a bit clinical and jittery in the backgrounds, especially with the messy foliage you find in Seattle parks. If you really want that high-end professional look for your sisters shoot, you might want to consider lenses that prioritize character over just raw sharpness. I would suggest looking at these instead:

• Nikon AF DC-NIKKOR 105mm f/2D
• This lens is legendary. The Defocus Image Control lets you literally change how the bokeh looks. Used ones are usually around 600 bucks.
• Tamron SP 85mm f/1.8 Di VC USD
• Be careful with the weight, but the glass quality is technically superior to the Nikon G for color fringing.
• Sigma 85mm f/1.4 EX DG HSM
• Not the Art version, but the older EX model. It has a softer, more romantic rendering that many pros actually prefer. Just make sure to test for back-focusing if you buy used tho. Older primes can be finicky on the D750 sensor, so you might need to mess with the AF fine-tune settings in your camera menu.

Regarding what #1 said about "Honestly, the 50mm can feel a bit clinical...", I totally agree. I spent years sticking to the safe bets because I was terrified of gear failing during a shoot. In my experience, the 50mm 1.8 is fine for snapshots, but for something like an engagement session, you want that compression that separates the subject from the trees. I remember my first big park shoot; I brought a lens that kept missing focus and I felt so unprofessional fumbling with it in front of the couple. Since you're on a D750, these are the two I would trust:

• Nikon AF-S NIKKOR 105mm f/2.8G IF-ED VR Micro: This might sound weird since it is a macro lens, but the glass is incredible. Over the years, I have used this for some of my favorite portraits. The bokeh is very soft, and the Vibration Reduction is a massive safety net when the sun starts going down in Seattle and you're losing light. It is basically bulletproof and fits your budget easily if you buy used.
• Tamron SP 85mm f/1.8 Di VC USD: If you want an 85mm, this is the one I recommend over the Sigma. Tbh, the Sigmas are great but they are so heavy and can be finicky with autofocus on older DSLRs. The Tamron has Vibration Compensation (VC) which makes it much more reliable for handheld work. The transition between the sharp areas and the blurry background is really smooth and creamy. I always tell people to value reliability over a slightly wider aperture... missing a shot because of weight or focus issues is the worst feeling... stick with something that has stabilization if you can.

Building on the earlier suggestion, I actually found the Nikon AF-S NIKKOR 85mm f/1.8G to be a massive letdown for the specific look you are after. I tried using it for a shoot at Volunteer Park last year and the backgrounds were just so busy and distracting... it looked amateurish. It was honestly embarrassing when I got the files home and saw how nervous the bokeh looked against the trees. If you really want that professional creamy separation on your D750, you should look at these instead:

• Sigma 85mm f/1.4 DG HSM Art for Nikon F
• Sigma 135mm f/1.8 DG HSM Art for Nikon F I eventually swapped my Nikon G glass for the Sigma Art series and havent looked back, even though the weight is a total pain. The Sigma 85mm f/1.4 DG HSM Art for Nikon F used should be right around your 700 dollar mark if you shop around. It handles that messy foliage way better than the Nikon primes I have used. Its basically a brick tho, so be prepared for some sore arms after the engagement session...

Honestly, the 50mm can feel a bit clinical sometimes. If you want that creamy look, grab the Nikon AF-S NIKKOR 85mm f/1.8G right now. Its way lighter than the Sigma and the bokeh is much smoother than the 50. If you dont mind the weight, the Sigma 85mm f/1.4 DG HSM Art for Nikon F used is a beast and definitely fits your budget.

Great info, saved!