toilet training - regression

hi

my daughter (nearly 3) was toilet trained just before the birth of her sister six months ago and has majorly regressed. intially it was just every now and again an accident would occur and now it has regressed to at least twice a day. i've tried pretending not to notice, then tried its ok its just an accident, to putting her in time out (which i thought had worked as no accidents for 4 days) and now we are back to weeing on the floor. shes not just weeing in her knickers but actually squating and going. i've even put her back into nappies but she wont wear them for pooing but will happily wee in them all day long? do i put her back into nappies for wees but allow her to poo in the toilet?? i'm feeling so angry at the squating that i dont know what to do. she doesnt squat every time so when it stops i think its fixed until the next time. i've always taken her to the toilet when shes needed to go and even try and put her on the toilet as much as possible. up until this point she has always been a happy pleasant girl thats always just wants to help and please me but now shes driving me mad.i love her to bits and dont want to be angry at her.

i went back to work 3 months ago and her sister is begining to be more active so i'm guessing this may have something to do with it.

please help