First of all a and a
of congratulations. Put everything else aside for a minute and what do we know? We know that you are pregnant with a sibling for your twins that you clearly want. that's really good news and it will be good news for the rest of your life. The stuff about jobs and money is important now but this will pass. your child is forever. CONGRATULATIONS 
Now then everything else:
DON't TELL work for a bit. You have no obligation to do so and don't need to deal with that stress just now. They cannot dismiss you for being pregnant and they will know that. Hopefully they are decent, fair employers who wouldn't dream of screwing you over this but maternity leave has a bad effect on some employers and leaving it as late to tell as you can is usually a sound idea. When does your probation end?
Dh's job - there's another 7 months before your baby will be born and possibly a few weeks after that before your 6 week 90% maternity pay runs out so he has time yet to find a permanent job.
dd's starting school - this is good as it hopefully reduces childcare bills? ARe you entitled to tax credits and child benefit? Child benefit for three dcs is 188 every 4 weeks.
MIL - if you get on well with her then just tell her about teh baby and that you were in shock. I'm sure she will be able to support you. She's probably worrying you're ill or something. Honesty is best.
The funeral tomorrow - I'm very sorry for your loss. Funerals are always hard. Wrap up warm, keep hydrated and try to eat if you can face it. Those simple physical things will help you get through the day.
Have you told dh yet? don't be surprised if he goes in to 'OMG how will we afford it mode'. A lot of men do in this situation and it doesn't mean they're not welcoming a baby too. Just that they want their financial ducks in a row. Just keep saying 'the baby is not coming tomorrow' and pressing cups of tea in to his hand.