What a ridiculous situation op!
How old is your sd and own dc?
Any child older than 10 and in good health can easily and willingly learn how to put their clothes in a washing machine, turn it on, hang out and fold away clothes themselves if you go about it in a positive way.
As well as help keep the house clean and tidy, do some help with cooking meals etc. You don't have to be a martyr.
The fact you have put 60% of your £ into land part owned is ridiculous while you are expected to contribute 50% to household costs!
You need to see a solicitor for starters.
If your selfish dp is expecting you to do the grunt work for free then calculate the cost of a cook, housekeeper, childminder and babysitter and factor all that in when coming up to a more suitable £ contribution from him or else divide up the day to day gruntwork/childcare so that you have more time for yourself either to have some spare time for hobbies or earn.
The reason you have been taken for a mug so far is because you have let him.
For goodness sake take back some control in your life and be as warm and loving as you can to your poor sd but make sure you give your dp plenty of bonding time with her as he cannot expect you to take over his role of looking after her!!
Stop being a doormat. If you are one it's because you are allowing him to treat you like one. No one respects a doormat.