I really don't know what to say that's going to be of any help or comfort Squirrel. I too would have been furious about this sort of thing - to not be consulted about a 2 week stay - of a "child" with serious problems, and have it dumped on me with no notice - even if I had no other worries.
Your DP does come across as having a big "attitude" problem with regard to his kids. Yeah - they're his kids and he shouldn't have to ask "permission" as such when he wants to see them, but you are also part of the household and family, and it is only courteous to discuss arrangements for skids with plenty of notice, and consideration for you - that includes taking due notice of other stuff which is going on in your life if necessary. Step-children may be part of the family but the hard fact is that they do not form part of your day to day household, and therefore arrangements do and should be discussed beforehand, if only to take account of stuff like extra shopping, getting beds ready and so on. To expect your DP to do this is not selfish or awkward.
You know ..... maybe I'm barking up the wrong tree completely but IMO, it's almost like your being tested in regard to the skids - i.e. he behaves inconsiderately and tactlessly .... almost "daring" you to object - and if - quite rightly - you do point out that you expect to be included in plans concerning them at your home, you then effectively get accused of not wanting them, stopping him seeing them and so on. If I'm right, then I expect that makes you feel right at the bottom of the pecking order and nothing more than the chief cook and bottle-washer. And right now, you must be feeling sick to the bottom of your stomach with his crass insensitivity.
Please please try and put him, and the skids out of your head. Difficult, obviously, but if you want to spend this time remembering your Grandad and having a sob as and when you want to, then that's what you should be doing. I'm sure your own children are helping you to do this but if they're not always around, have you thought of calling the Samaritans, say, just so you have someone who will listen and will let you sound off. They're not just for suicidal people but for anyone who needs a shoulder really. Also, there's Cruse - the bereavement charity, who also have a helpline (0870 167 1677) .... they'll be able to talk to you if you need. I know that's not the same as having someone close supporting you, but make no mistake, these people will support you (unlike DP) and if grieving is what you want to do, by talking about it you hopefully should be able to put all the other stuff out of your head for a short while (as opposed to musing it all over when you've only got your own company, if that makes sense ?)
I think your friend may have been half right about people who're very close being unable to express their sympathy. However, even if you can't say the right thing, your actions shouldn't be "saying" the wrong thing either ! In other words, no way should he even have to think about having a 21 yr old druggie to stay right now - so why ask you ? Why put you in that impossible position, where you can't win whatever you say ? You know, I'm sorry to say this but DP actually seems jealous that he and his bloomin' kids aren't at the forefront of your thoughts right now - it's pathetic, and I feel very hurt on your behalf.