It's hard because when you are in the middle of it, it's so fucking frustrating. Especially when you have a baby, new house, not much sleep... and they seem 'old enough to know better'
. However, when you are on the outside looking in they just seem like tiny tots struggling with so much change, just wanting to be mummy's baby again, wanting to be mummy's only baby again... wanting to be cuddled and fussed over like the baby. They don't really understand their feelings and they have so few ways to communicate their needs, that they do it by regressing and demanding :(
I would try, first of all, a bit of love bombing. Try to make her feel very special all day, lots of hugs, kisses, comments and try not to make too much of the baby. When the baby is happy playing, eating etc don't make a fuss. Give her some Big Girl 'jobs' and rewards, tell her how special it is to be the Big Girl - MUMMY'S BIG GIRL and how boring it is to be a baby. (Your 10 month old wont know or care about all of this). Make a big deal of how much she can help you because she's your bestest Big Girl and the baby needs it's nappy changed because she's a baby and not a Big Girl. Do things like have a 'carpet picnic tea' with her once the baby is in bed because on ly Big Girls can do that. Choosing what to have for dinner or where to go or whatever it is - things only Big Girls can do.
Try to get DH onside and both of you do the things she asks, even if she can do them herself. She needs to trust that she's not being replaced by the baby in your eyes/affections.
It's all very Rah Rah with a big smile - but generally it does work.
After a while she will see that being Mummy & Daddy's Big Girl is much better.
When she has a tantrum, try holding her against you. Very firmly, but lovingly, not angrily (it's not easy, just do your best) and talk quietly and calmly to her. Tell her that you understand she's upset and that you will help her to sort it out.
Put the baby somewhere safe and if the baby cries, the baby cries - it wont hurt her to be left for a few minutes.
There is no harm in trying this way first is there :)