jellybeanjean, congratulations on becoming a grandmother to twins. It is obvious from your post that you are torn between wanting to go to your daughter and help her with the new babies, and the commitment you already have in caring for your physically disabled husband.
Helping care for newborn twins could be hard work and although it might seem more enjoyable than caring for an elderly spouse it does not exactly give you a break. Whether you stay at home with your husband or travel to be with your daughter you are still in a carers role.
Many of the suggestions made by other posters, whether to bring in extra help for your daughter, or help for your husband, or residential/respite care are all very well but that is presuming that there are either free local authority care places available, or that you/your family can afford nannies, carers, care homes etc. I know IVF can be financially crippling for some families and not everyone qualifies for free NHS treatment, so don't know whether your daughter and her husband would be able to afford help or if it is just because your daughter wants her mum there. It's perfectly understandable that you would want to be there for your daughter, to help and to share these first few precious weeks of their lives.
If your husband requires medical aids, lots of medication, personal care etc I can see it might be impractical to consider traveling with him - and even if he did want to go it would be twice as much work for you with him and your daughter and her twins. Was this sprung on him recently or has it been on the table during your daughter's pregnancy? It may have come as a bit of a shock to him if he hadn't even considered you leaving him for a few weeks or him having to have other carers or to go elsewhere! You say the suggestion didn't go down well - by this can we infer your husband is capable of decision making? Is his a knee-jerk reaction, stubbornness, wanting care to come from you and you alone, fear of staying elsewhere or having strangers coming in to his home?
I'm sure you have considered all these types of things and hope whatever decisions or compromises you make can work for all of you. I wish you well and congratulations again.
On a side note, I sometimes require more care than other times, and depending on his reason for wanting/needing time out/time away I might not be happy about it but would work with my better half as his wants, needs and wishes are important to me - and, a bit selfishly, if he is rested, able to have time away every now and then he is less likely to resent all the help he does give me, be refreshed and more relaxed etc. I would never begrudge him a break and am acutely aware he would not leave me for trivial reasons. Carers are the unsung, often-unpaid heroes of our society = providing round-the-clock care from a place of love - back to that age old question of 'who cares for the carers' - everybody needs time out sometimes.
and
for your daughter too.