Even though I currently have three children, I was a lot like your sister. We had two boys spaced three and a half years apart. I really wanted a third but I had a really hard pregnancy and birth with my second son, so we had some fears about trying again. Time went by...so much time that by the time my second son was six, we decided to forego the third, buy a second house and I went back to work part time.
Well four months into living in our new house, I found out that I was 16 and a half weeks pregnant with a baby girl. (Because of the difficulties getting pregnant the second time plus with post pregnancy issues; I did not think getting pregnant would actually happen naturally especially because I am 42 and my periods are not consistent)
So, the answer to your question, 'Yes, I would have regretting not having a third. She is the light of my life.' With that said, we have lost the freedom we had by having just the two boys , who are now 10 and 7. Money is tight, I am staying home again (happily but less income coming in). It's hard to dive into the day knowing that each child needs something far different from the other. And yet, all she or the boys need to do is smile or tell me they love me and I am reminded that all of the work and stress is worth it.
If they are unsure, they probably should give it some time and see if things happen without trying so hard. I wouldn't worry about her age, lots of mothers are conceiving and having healthy pregnancies and babies in their 40s. Yes, the percentage goes up that things can go wrong but it is just that a percentage.
I wish the best for your sister whatever choice is either made by her family or by God. Hopefully she will find peace in her heart as her family's story unfolds.