Do not use Rome2Rio! It can be wildly off-base. There are many companies operating trains in Great Britain, and they all sell each other's tickets--at what I believe is exactly the same price. I used to just go to https://www.nationalrail.co.uk/, look at all my options and choose the one I preferred. NationalRail then automatically transferred me to the website of the company operating my train so I could make the purchase. But then I learned on this forum that sometimes you can save money by buying two separate tickets rather than one, and that ScotRail does that automatically. (There are also some private companies that sell split tickets, but you normally pay extra when you buy tickets from a private company.)
In most cases you can buy rail tickets shortly before you want to travel, but you will in that case pay full price, and it can be very high. (Take a look in the morning at fares for the current day.) If you want to travel that way, it's possible a rail pass will be a smart purchase for you. I you are willing to buy ahead of time, it's a different story. If you're moving very rapidly through Great Britain and don't need a pass that covers a lot of time, maybe it will make financial sense for you.
There are no time constraints on use of a Britrail pass. As long as the train isn't full, you can take the train with either a purchased ticket or the rail pass. It's a good idea to get a seat reservation when you're a tourist traveling some distance. It's not pleasant to have to stand in the vestibule until a seat becomes available. There are no limitations on reserving seats with a rail pass unless the train is completely full. (It's different in France, where there's a quota for rail pass holders on some trains that can prevent travelers from taking their desire train even if there are still tickets available for the train.) I believe seat reservations are free.
I must warn you it's not rare to board a train and find that none of the seat reservations are posted. When that has happened to me, there have always been plenty of seats available.
Since you're seniors, you have another option, in addition to the Britrail Pass and the Two Together Rail Card I mentioned in my earlier post. There's also a Senior Rail Card available for £35. Each traveler requires his or her own Senior Rail Card, unlike the Two Together Rail Card that covers two people. But the Senior Rail Card has the advantages of being without a time constraint. You can travel during morning rush hour on weekdays if you want to. [<< That is incorrect; see isn31c's post below.] So you could use a Senior Rail card and your sister and her husband could use a Two Together Rail Card. The total cost for those rail cards would be £70, and you'd all be able to buy tickets at 1/3 off. But your sister and her husband would have to be sure they always travel together. It would not be possible to use a ticket purchased with the Two Together Rail Card if one of them decided to rest in the hotel one day while the other went off on a day trip.
I've used a Senior Rail Card on multiple trips. It's an easy process to buy one at a train station. I believe they can also be purchased online, but I've never done that. You don't have to buy the rail cards before you buy train tickets at the discounted price; you just have to have the rail cards when you board the train.