From various forum posts and some YouTube videos, we got same feeling as your sister.
However, we visited the palace with the nuisance of timed ticket and boy we felt stupid for not taking the advice from our own research.
First of all, ground ticket is not just a ground ticket. You can visit 80 pct of the palace terraces including cafeteria. All those Instagram photos can be taken with grounds only ticket.
Interior was a huge let down.
Grounds ticket is a misleading name. It is a palace ticket without painful interior visit. It is half the cost and has no tension of timed entry.
The 3 euro green bus transfer to the top was worth it to preserve time and energy for other attractions
Now the problem was that I could not figure out how to buy "palace tickets without interior" online in advance
But if you go in March, it might be easy to buy from machines at the entrance or even from the counter.
Only time we encounter crowd was this stupid walk through of the interior.
Another thing:
Would certainly give Regaleria much higher preference than Moorish Castle. RS in his book seems to give lot more attention to Castle than the Regaleria. But if you read posts on this forum or TA, folks prefer Regaleria over Castle by huge margin.
Note that given the long winding one way road and terrible traffic, uber/bolt drivers do not like to come up to pick pax for short journey back to town. But if someone is getting dropped off , it is easier to get a driver assigned.
This definitely going to happen more likely at the main gate of Pena Palace than at Moorish Castle.
TLDR:
1. Buy the cheaper untimed palace exterior only ticket
2. Skip Moorish Castle and spend time at Regaleria or complete the sintra-cabo de Rocca - Cascais circuit in leisurely manner
3. Avoid expensive tours and inconvenient train/bus. Use cheap Bolt/Uber directly from your hotel in Lisbon. (Costwise it is no brainer for a family but even with 2 people saving of time and energy makes it worth it)