Lisbon lands itself really well to walk and wonder. Here is how I would start showing the city to someone I love.  I am sure your sister and husband will find their ways to explore this gorgeous city. 

DAY 1 ---- Get a good taste of the city overall /best views

1. Park Eduardo VII Observation Deck - Maybe start from here. Beautiful viewpoint here that looks down to the tree lined park, Marquis de Pombal below and all the way to Avenue de Liberdade and to the water - Tagus Review. Amazing panoramic view. You may want to come back to this park for lunch or dinner as there are great restaurants in the middle and around. 
   

2. Praça Marquês de Pombal - Biggest roundabout in the middle dedicated to their national, liberal hero who rebuilt Libon after the big earthquake, influenced by the age of enlightenment.

3. Avenida de Liberdade (It’s their Champs Elysees) - Beautiful tree lined big street with Cartier, Gucci etc. as well as various monuments and embassies. Pavements are amazing!. You run into cafes between stores too. 
   

4. As you head towards the water, you want to go towards Praca de Commercio below. Huge square by the water. Cafes restos....
   

      6. Here you can enjoy the views of Tagus, the bridge and 

          

      7. Perhaps walk by the water to eat lunch at TimeOut Market. Food Hall with tons of local gourmet options! Sardines done local way :). If they care to sit outside to eat as opposed to indoors ar Timeout, then step out and go diagonally across to find Cafe Janis - superb!