Windsor Castle and Stonehenge Tour?

I would give Windsor Castle and the town of Windsor it’s own day.
I have been to London at least six times and haven’t gotten to Stonehenge yet.

To me, this multi-venue tour is too much in one day. You are spending a lot of time on the coach just to get from place to place.

I have been to Stonehenge a number of times but never from London. I've gone out to Salisbury and spent at least 2 nights, then traveled to Stonehenge on the local shuttle bus out to the stones. Coming back it stops at Old Sarum which is interesting.

I've also been to Windsor Castle a number of times and have done it as a day trip from London. It's very easy to get to by train.

I'd encourage you to give each of these historic sites their due and not try to cram them in to one long, exhausting day with very quick visits to each.

I think deferring Stonehenge to your next trip is fine. As mentioned, I've been a number of times and I really enjoy it although you'll have many say they did not think it worthwhile. I think it's worthwhile when you have enough time! The big problem with UK is that there is SO much to see! I've got an ongoing list of things I want to see that I've not been able to work in on previous trips. It's never-ending, lol.

To answer your question directly it is not really practical to do both in a day by public transport.
The reason is really one of logistics.
Windsor Castle doesn't open until 10am. To do it decent justice say you are there for 3 hours, that's got you to 1pm.
Whichever train route you took (Central or Riverside Station) there are several changes of train to reach Salisbury for Stonehenge. The bus from Salisbury station runs hourly, so you want to be no later than on the 3pm bus, then the last one back.
It's all but impossible to achieve that schedule.
If you did you would miss Old Sarum and seeing more of the City of Salisbury.

If you search this forum you will see several recent threads about the merits and the history of Stonehenge.
I guess the question is how much do stone circles mean to you/your sister, as against Windsor. Or is it more that Stonehenge is a "tick list" place to go to, and please don't think I am being dismissive in saying that- I don't mean to be. Would missing Stonehenge lead to family discord in the future?
The feeling many people have is that there are other stone circles which are less regimented. less crowded, maybe less touristy to visit, which convey more of their meaning and spirituality.
Both places deserve a day of their own.
Which is your priority?
If you really can't decide then one of the tours really is your option. It will be a bit of a whirlwind and possibly not give you all the time you would wish at either, but I'm not going to discourage you. It's certainly better than trying to add on Bath or Oxford as well on tour.
You would be on a bus, so can sit back and relax on the 3 legs of the journey.
There's many questions here which I can only give guidance to.

The definition of a vacation includes the concept:
an extended period of leisure.

Trying to jam in a site or two because you feel you must doesn’t seem leisurely.

Is viewing of large standing stones or a royal castle more important than the other? The answer is based on your interests.

Thanks everyone! We've decided to just stick with Windsor Castle for the day.

I took a bus tour from London and saw both. It’s a long day, but I was far from being tired. We too only had one day, as we had a two day layover in London.
If you want to see both, see both.

I did this on my first trip to London, and for a first time solo traveler it suited my needs perfectly. My guided tour included Cambridge and we only had about an hour there, would have liked more, sandwiched between Windsor where we had 3 hours and that allowed time to explore the castle, chapel and grounds--my favorite thing of the day was the glorious doll's house--and a bit of the town, and Stonehenge was last, about an hour there to gaze at the stones. It's an iconic and fascinating place, and it sounds like combining it with Windsor on a guided tour would work for you.