I've not "been there", but I'd like to add my 2¢ worth, after reading some of the responses. Please don't assume that your husband's daughters will be ecstatic to "help with the baby". Not all girls are so inclined. Some are, true. But NOT every one. I certainly was not. Neither is my own daughter. And I would go so far as to say that if you push the issue, they will think you view them as babysitters, not as the daughters that your husband loves who are in their dad's house. If they show an interest in the baby, great! If they don't want to become instant babysitters or "helpers", let it go. In either case, let THEM set the pace.
Spend plenty of energy being enthused that they are there to spend time with their dad, and show an effort to provide fun activities for THEM - not focused around the baby. They will already be dealing with displacement issues (dad's remarriage and NOW a new baby, too), so show them how happy you are to have time to get to know them.
It might be a good idea to talk with your husband about a list of things that you can do while they are visiting: an afternoon at the beach/pool, taking in a matinee, taking them for a manicure or shopping, a trip to the bookstore to buy a book or two, a netflix schedule of movies to watch, etc. Then, when they arrive, post the list of "ideas" somewhere where they can see you have been planning how to enjoy the time with them... and ask them which things they are most interested in doing.. then try to squeeze their favorites in at some point during their stay. That way, items that are not in the budget are not on the list, but they will not feel awkward about requesting something fun to do. If necessary, have a trusted family member/friend babysit for the afternoon so you can take them to the beach or the movie. Don't let the baby become an excuse for not doing fun things with them. Maybe they even would enjoy camping out overnight in the backyard with Dad on the weekend. You could all enjoy s'mores, etc... but when the baby is "done" for the night, you can take him inside and put him/her to bed and carry the monitor outside while the rest of you stargaze and identify the constellations. That's great family bonding and dad/daughters bonding time. You could even ask for their input on meal planning.... Write out the general meal plans (steak, chicken, pasta, etc with a separate list of side items: cole slaw, salad, mac n cheese, green beans, broccoli, etc) and ask them to glance over it to see if there is anything truly objectionable (maybe one kid absolutely can't stand squash or gets stomach upset eating something prepared a certain way) or anything they like that is missing from the lists. That leaves it up to you to figure out meals, but lets them know you are interested in making their visit enjoyable.
And make a BIG deal out of the birthday girl.... let her pick the cake, or the restaurant for dinner, etc. If your husband didn't see the other daughter on her birthday, have a birthday cake for her on another day (her 13 1/2 birthday or something). Maybe even take the girls shopping (to let them pick out the other one a birthday gift).
Again, just my 2¢. The baby is YOUR biggest event in life thus far... am I right? But it's NOT theirs. Just put yourself in their shoes by imagining you're at a party/picnic, and you are stuck talking to the person who only is interested in a subject that you couldn't care less about. How tired of being around THAT person would you be?! Don't let yourself be that person to your husband's daughters.