AS a mom of twins (born premature) just 19 months after their sib I can relate.
For the other moms on this thread - one of the things that was hardest for me having 3 small children was that I was not able to spend quality time with the twins as I did with their olders ibling. It does not sound like a big deal but it was perhaps the hardest adjustment. Our oldest DC had all our attention but the twins got 1/3 at best. The other hard part was with my hands nearly always holding a fussy baby, often with her sister crying in the background was that the number one phrase I repeated to the older sib was "no, don't do that' and #2 was 'sorry, but no right now'. I opted for preschool for him to create a better environment. I was lucky and both DDs thrived after the first few months and it was once again enjoyable for everyone to be home.
OP - I would opt for a babysitter that could come to the house. Perhaps you can leave her with teh twins for a few hours each day and enjoy time with your older child.
Good luck.