M.B.
You will have a lot on your plate, that is for sure....
Even though some people don't like Michelle Duggar, I think she has some really great ideas..... and one of those is to incorporate all the older kids in assisting with the younger ones..... (When a new baby is born, she assigns one of the older kids to be their "buddy" ...)
Your 12 and 11 year old are old enough to be very helpful with the babies... I'm not meaning to have them completely responsible for them, but they can do a lot to keep the 5 month old occupied, and change diapers, and feed.... they can also help a lot (with supervision) with the upcoming twins. They can also be a really big help with the 5 year old, by incorporating her into the baby care and playing with the 5 year old so she doesn't feel so left out.