lilymolly - congrats on DS
I am sure her behaviour is due to little DS, however, in my opinion it is still not acceptable.
Carrots AND sticks!
I would be having 'consequences' for any naughty behaviour because I think if you let it ride it's much harder to rein it in later on and I believe in fast, firm action from the beginning. I do think you need to find something that really means something to her though - of all things, no playing with toys in the bath, just getting washed and out is the BIG consequence in our house (LOL, I know, hardly time in cell block H is it?!), naughty step, toys taken away, toys thrown , being sent to her room, no pudding (naughty I know, but worth a try) etc don't have anywhere near the same result - so just find the lever you have and use it It may, or not, be banning Peppa Pig - not because it's the cause, but because she is copying Peppa's annoying habits (we now watch very little Charlie and Lola, because LO was mimicing Lola's less than charming attitude and whiney voice and I don't have Charlies patience!!).
I think that they benefit from their rules NOT changing... you wouldn't put up with this before, so why would you now? If the rules change she has to use a lot of energy up (both hers and yours) to see which rules have and which rules haven't changed!!
But also REWARD REWARD REWARD any and all good behaviour - so stick to the rules, say no and mean no etc, but tell her she's being good when she is, (as others suggested) get her involved in anyway she is interested in helping and tell her what a GREAT big sister she is, really overplay it Have Big Girl time when (if??) DS is sleeping, make sure she knows she is still Mummy & Daddy's best girl
I am sure you are doing all of this already, but just wanted to help you think through it a bit with someone who is also 'strict'.
Godd Luck and remember my favourite muttering when things are tough...this too shall pass, this too shall pass, this too shall pass.... oink oink