I think most kids feel that the youngest gets all the attention, and that may be more so in your case because your daughter has a disability. And perhaps you have subconsciously (or others have) pushed your boys to "be the man of the house" since your husband's death - which people mean as a compliment but it gives children an adult job and says there's no time to nurture them as much now.
I so desperately wanted a daughter and just assumed I was having one (lotta girls in the family), so imagine my surprise when I found out I was having a boy. I think you're going to have to do what I had to do - truly examine why your daughter is what you "longed for" after 3 boys. Doesn't that tell you that, somehow, they weren't enough for you? Their maleness didn't fulfill something in you - and that tells them they are not equivalent. You can keep stating that you love them all the same, but either you don't quite or there's something in you that those boys know they can never satisfy. That's got to feel awful to them.
We need to ask ourselves why we pigeon hole our kids by gender, why we use terms like "girly girl" or "mommy's girl" (or, "it's a guy thing" or "boys will be boys"), why we repeat those sexist mantras of "sugar & spice & everything nice" only for girls, and so on.
Please get some counseling for yourself. You have had a terrible loss in your husband, and you've lost a part of your boys.
And do ask why you "cave" in to your daughter's tantrums, and why your boys' needs don't matter as much. Maybe one of the reasons she has a hard time with change is that you don't require it of her. It's really okay for a child to be frustrated and to learn to deal with it. It's okay to cry and be ticked off and learn that the world continues. You're handicapping her more by allowing her to get her way all the time and not learn patience, and you're teaching your boys (totally unwittingly, I am sure) that men's emotional needs aren't as important as those of women. Don't do that. You're teaching them how to treat women by building resentment and squashing their feelings. I know you don't mean to, but that's the result. You give in to your daughter because it makes YOU feel better, not because it's really better for her. She needs to be left alone sometimes, she needs to be forced to deal with frustration, and she absolutely needs to sleep on her own and not on you.
Your sons have given you a great gift by telling your sister how they feel. You made a start by talking to all 3 boys. You started to devote time to them. So why, please, won't you continue by proving to them that you mean what you say? Get a sitter, trade with a friend, anything, so you can do age appropriate things with each child, including splitting the twins up sometimes and not always doing stuff with all 3 boys together. Don't make them into rebellious teens who look elsewhere for acceptance or ways to numb their pain - that's a terrible road to go down.