This is probably the most difficult dilemma I’ve ever seen on this forum, I really feel for you. What a distressing situation for your whole family and especially your young niece.
I am similar age to you, but I’m single, childless and always wanted children, so of course my thoughts are swayed. If it were me/my niece, I would take the baby. Of course it’s far from ideal and would be exhausting but I would want to keep that family connection and I’m physically and financially able to do it.
If I were in your position though, the one thing (probably the main/only thing) that gives me cause to rethink, is the potential within the next 18 years, while this baby is still a child/teen, that both you and your DH are gone, and the responsibility to continue raising them falls onto your sons, who would still only be in their 20s/early 30s. That’s a heavy responsibility for them, and they may decide they don’t want to or might not be able to.
I know plenty of people who have remained fit and healthy through their 50s, 60s, 70s, but I know an equal number who lived a fit, healthy, active life and in their 50s/60s got cancer or other serious and unexpected medical problems. It’s a real consideration to think of your sons - not so much gaining a baby sibling now, but becoming parents to a teen sibling later on.
Taking that scenario into account, the kindest and most practical way might then be to have the baby adopted from birth.