I'm with you there, @Dropthedonkey, this needs to wait until after the funeral, I think. Then, when things have calmed down a bit, you'll both be able to look at this a more calmly.
Sorry for your loss, OP, and I'm sure you don't need this on top of your grief. But there are some things that might be relevant, eg:
Who is the executor of the will? If there's a solicitor appointed to do it, it will be up to them to explain to your sister that, in the absence of any other will made after this one, what's specified in this one must be followed. If no executor has been appointed, it might be best to appoint one.
Will the value of your DM's estate be above the threshold for inheritance tax (£325k, although if the house was jointly owned with a spouse, it can be up t to double that)? If it is, the house may need to be sold to pay the IHT.
Who has the actual will?
Hopefully, in a little while you'll be able to have a calm discussion with your sister about whether she gets a mortgage to buy out your share and keep the house, or if it has to be sold and the money shared between you.
This is the last thing you need when you're recently bereaved, and I hope that it's resolved with the minimum of hassle.