I guess I wouldn't call this sibling jealousy yet, but I hear your fears. At that age, my girls were day-care children as I worked full time. They were, therefore more attached to other adults, as well as mom, and they had a network of other children in their lives at the sitters.
I believe children are very good at telling you what they need. Obviously your daughter feels she needs to cling when Mom might be doing something else. Perhaps a way to deal with this is not to do so when it happens -- Instead, have Daddy spend extra holding time, so the need to be held moves to her other parent. That way, later, when you are holding or nursing the baby, Daddy can be holding her, or playing games with her, and she will feel treasured at the same time. You can do this with friends, too. They can ask if they can hold your daughter, and when she says it's okay, then they can ask if it's okay to give their baby to Mom to hold so their arms are free to hold your daughter. Let her make those decisions, and honor them when she says it's not okay for her mom to hold that other baby. The more control she has over her environment, the more relaxed she will be about Mom holding some other child.
(I used to use nursing time with #2 as a time to read a book to #1. We kind of all cuddled together as best we could and we read LOTS of books !!)
When around other babies, another thing you could do is ask if she would like to hold the baby, rather than you holding the baby -- (assuming the mom is cool with this) You pretty much support the baby, but let her get settled in a comfy and safe place, then lay the baby across her lap let her "hold" the baby, and touch it, and talk to it. Praise her for being a big girl, and being big enough to help moms with babies. (You'll also need to be sure she understands that she is helping, and not old enough to hold babies all on her own.) Babies tend to be kind of boring, so don't be surprised if she pushes it away and heads off to some other game . . . Just be prepared. :-)
At diaper time, she can help by handing baby wipes, and diapers to you, and she can stick the tabs down (unless you are using diapers with pins, like I did !!) When it's something you and she do together, it can be fun for her, and it might help her to mature a little.
The other thing is to remember that these are concerns. They aren't a reality yet, and age 2 is a growing year in which kids generally begin to attempt a level of autonomy. Praise her for the appropriate autonomy and understand and deal with what is inappropriate. But be sure she feels it is safe to become more autonomous, doing big girl things, etc. We had a golden book called, "The Big Enough Helper", which was about a kid helping Mom with the laundry, but we read it a whole lot cuz the girls loved it. The story line is about a kid who wanted to help, and Mom was frustrated, and told her she was too small, etc., to help, but a guy in the laundromat asked her to help entertain his son, and she did. Then she folded socks with the guy -- and Mom ended up giving her a big hug and apologizing for not thinking she was big enough to help. Maybe that's why the kids loved it so much -- cuz it was Mom at fault, not the kid !! But the idea is to become a "team" of sorts with your daughter, so she will consider taking care of her little brother as something you do as a team, with different ability levels for sure, but not something that divides the team.
When our #2 was born, I discovered #1 wanted to sit in the little car seat, play with the baby toys, etc., and we generally allowed that to happen when the car seat wasn't in use, etc. She wanted to be a "baby", and that was okay, as long as she wasn't pushing the real baby out in order to use the baby toys. #1 child can provide lots of entertainment for a young sibling who can't sit up, because #2 can watch #1 move all over the room, listen to them talking, and stuff like that.
the other thing we did, that worked very well, is that we selected a toy (a wee little weeble doll house, which will date my children --ages 27 and 24 now) that we thought our 2 year old would really LOVE. And when the baby came home to live with us, her older sister received a special gift. This helps, too, if people are bringing baby gifts to you. That is the part I found hardest on the older sibling -- the baby is so "exciting", and everyone wants to hold the baby, and everyone wants to give it gifts. You can help in some ways, by handing off the baby, and using that time to hold your older child, and let your older child unwrap the gifts and, then maybe your family will come with a small gift for your daughter instead of a baby gift. Those moments are very special, even if you "Plant" them, because they will meet a need your child at 2 will probably not be able to express without acting out.
Good luck !! I am a parent who actually LOVES 2 year olds !! So I'm probably a little wierd. I love the teenaged years, too, and most people fear and/or hate those years !!
barb