Seconding the notion that December weather is fairly unpredictable. I was in Lisbon from Dec. 18-24, 2021 (not last year). The first two days were around 60 degrees with abundant sunshine -- absolutely perfect -- but the next four days were very rainy with only brief bursts of sun (but also not much in the way of total downpours--fine to be out most of the time with a light jacket). Alas, this was 2021 in the height of the Omicron outbreak and I was extremely averse to doing anything indoors because I had to pass a COVID test to get back to the US, but Lisbon has plenty of great museums, aquariums, shopping, food halls, etc., that could help fill a rainy day. There were almost no tourists when I visited (but, again, COVID), and I had the Sao Jorge Castle and much of the Cascais-Estorial boardwalk without any crowds at all. I personally think it's still a nice time to visit. Prices are a little lower and the weather is still likely better than most places in the US, although maybe not California, where it looks like you are from.
For what it's worth, I saw Sintra on a separate visit in May 2018, and we had a wonderful time with a guide who drove us to Sintra and then Cabo da Roca, handling all of the ticketing and prearrangements. I will add, though, that people I know who have gone since have found it to be fairly miserable. It seems like they haven't found a way to adapt to the maddening crowds visiting. You could perhaps consider Evora in place of Sintra. I found Cascais to be an absolute delight, and it's only a 40-minute scenic train ride along the river/ocean from Cais do Sodre station in Lisbon.
Hope this helps... Bottom line on the weather is you just don't know, especially in December.
