I don't have an older child I have a younger infant (4months) AND 1 1/2 year old twins, a whole different challenge (but I'm glad I had twins first). Twin parenting is about survival and doing your best. They'll love you no matter what because you're THEIR Mommy :-) This is your new family, and your parenting will have to adjust. Don't be hard on yourself! They're going to cry, unless you can hire a nanny or have volunteers come in 24/7 to help you, this is inevitable with twins. I am amazed at how little my 4 month old baby cries, because generally I can quickly attend to his needs and he is breastfed/cosleeps part-time. The twins cried a lot for minutes at a time because I had to triage their needs and act accordingly based on urgency. I had to breastfeed/PUMP/Formula feed the twins and gave up the breastfeeding/pumping routine after 2 months because they'd scream while I pumped and one would scream while I fed the other at the breast and vice versa. It was too stressful. We coslept off/on until 6 months, but after that they wanted nothing to do with my bed and think it's a party if we bring them to sleep with us.
The bond with my girls, in my experience, has grown stronger as they grow up (they're currently 1 1/2). Each one is so special and has her own unique personality and it's awesome to watch them grow as individuals and together. One thing that helps me with bonding is getting each twin out individually to spend quality alone time with Mommy at least once a week. I even babywear them when they're in the mood for it on our adventures, so if you cannot babywear your infant you can babywear later on when you can get them out separately. We go to free classes, paid classes, the mall, target, dinner, or grocery shopping etc. My husband feeds the little one a bottle of pumped milk while one of my girls and I spend time together.
Anyway, good luck. It does get easier, I promise! Just be easy on yourself, you are doing the best you can!