It sounds as if your stepdaughter has hit her limit on changes and is getting her anger and frustrations out at daycare.  There are several options:

1-Consequences.  If she has a bad day at school, take away a privilege at home. If she has a good day at school, add a privilege.
2-Praise her for being the older sibling and give her additional responsibilities in the home and family.  (Some of these responsibilities should be very fun.)  Remind her that these activities are not for babies- just older kids :)
3-Purchase some barbie dolls (or other dolls) and let her "act out" her blended family.  Spend some time with her listening to her as she vents her frustrations. You don't have to solve her issues, just listen.
4-Give her paper and crayons or markers and let her draw for you each afternoon.  She may be able to vent her frustrations this way also.

Continue to provide your stepdaughter with reassuring information- you love her; new additions to the family don't change her importance; it's O.K. to love mommy, daddy and step parents; and any other relevant information that will help her understand all these changes.

Kindest Regards,
Shirley

Shirley Cress Dudley, MA LPC
Founder of The Blended and Step Family Resource Center

Here are a few suggestions that I have used as a child care provider. When she is having a tantrum at day care, have them remove her from the immediate area and take her to a quiet place. This place should be safe for her, but not where the other children can watch her. Have them tell her that when she is ready to talk to them, then she may come out. Have them Walk away, but keep the child in sight,  do not talk to her. When she starts to calm down a little, ask if she is ready to talk. This may take awhile at first. But those times will become shorter and shorter. Tell her that her behavior is not appropriate and you do not want to see it continue. This needs to be her choice, not yours. Tell her that she is a big girl and you know that she can make these big girl choices. Have them praise her for any attempt that she makes to do what you want her to do. Another thing that could help would be for you to send a picture of the family with her to day care and tell her that she needs to keep it in her cubby or whatever and that she can look at it when she starts to miss you, but that she can not carry it around all of the time. I have also had the parents bring a special small stuffed animal for her to rest with. This is not something to use all of the time, this is a special gift just for her to remind her of how much you all love her. You can also have a one on one special time set aside, just for her with mom or dad, such as going out for Sunday breakfast or maybe a movie night that she earns when she makes good choices at day care. Be sure to tell the daycare provider what you are trying so that they can remind her to make good choices. They could offer a sticker chart that they use to help her remember to make a good choice. She could put a sticker on every day that she doesn't throw a tantrum and at the end of the week, she gets her special time. I have used them all, sometimes all together. Remember to praise her for all of her good choices and try to diminish the bad ones. We call this, watering the flowers and not the weeds. It really works. Good Luck.