YABU. A single 5 yr old is pretty self sufficient compared to TWIN 3 yr olds to an extent and having one child to entertain is much easier than 3, 2 of which are very reliant toddlers probably still in nappies or are training?
I think the problem here is that your expectations might be a bit too much OP. I think Gran is saying it light heartedly and telling you what she is willing to try and cope with (not saying age is a barrier but it could be?) and your response is all stay or none stay 🙄 I think that is quite sad for DS and gran actually.
I think you're not a very reasonable daughter to expect more than what is on offer and possible factors around why Gran is only willing to take on one child this time. I used Gran's age as an example earlier but it could be any reason, it is really not a negative at all.
The lovely gesture to have one of your DC stay away for some one on one that he will no doubt cherish when dear gran is no longer here and you're considering not letting DS stay because the twins (who will in no way be affected by not staying) really because of your own feelings about it? Give it a go, you might become custom to DS staying away..!
Why not let DS stay and you do something girly and fun with your girls while he is away? Do stuff you might not always get to do with them as DS is always there and has different interests to his sisters
Here Is an Idea for you:
Make A unicorn or Princess Den downstairs and pamper yourself and the twins!! Fill 2 sink bowls with warm glittery soapy water for them to put their tiny feet in, give them mini mani/pedi's, cucumbers on their eyes, baby lotion or whatever you use for them for their 'facials' (hand towels tied with a band should work as a hair wrap for them) and ENJOY joining in and putting a face mask on yourself and letting them do you a foot massage ❤️one foot each❤️ have a ladies night with your little ladies. Put on a sing along movie and have ice cream with sauce and or popcorn!
Enjoy it - It can be fun, requires a little imagination.
See it as an opportunity to do other things and a chance for your boy to be the center of attention once in a while - it isnt easy being the eldest sibling, I mean .. what is he doing while you are busy with them? What COULD he be doing while you are busy with them? Its a win for all of you if you really think about it.
I think you should agree to DS staying with Gran and be grateful for the chance to be able to just focus on the twins and yourself (I am only just managing to find time for myself as a mum of 3 so I know it can't be easy with twins or girls!!)
Good luck OP