You aren't being unreasonable. BUT I would think long and hard, and think beyond yourself, before trying to get pregnant and having a newborn well past 40.
I am not judging older parents, I am coming at this from the perspective of someone who nearly died putting my body through "one more", and then had a life altering brain injury when she was 14 months old, so my daughter has only ever known being a carer to a disabled mum.
My oldest son's father also passed away last month aged 46.
The chances of something happening with your health either during the pregnancy or while they are young, is higher the older you are.
And if something did happen your twins would then end up having to parent the tot.
I had a 10 year age gap and was desperate to give my son a sibling, having been a lonely only child myself, but he couldn't have cared less and had no interest in a baby. It's not necessarily something people want when they are starting high school.
Also, having been through many rounds of IVF to have my daughter, I will say that conceiving and carrying a healthy baby to term in your 40's is against the odds. There is a lot of potential for heartbreak. If you start trying to conceive and fail, something you were initially on the fence about can absolutely consume you, like it did me. It can bankrupt people and destroy relationships.
The Janet Jacksons and J-Los popping them out at 50 are invariably using either frozen or donor eggs. They have also never worked a day in their lives and have an army of chefs, nannies etc. it's not real.
Additionally there are the cost and resource issues for your family, as your standard of living is likely to drop considerably. You start having to choose who can do a sport, who has to miss their school away trip. Holidays and lie ins disappear.
So if it were me, I would look into other options such as fostering perhaps, or getting a dog.