About 1 year ago, myself, brother and mum planned to go on a holiday to NYC together in Xmas. Now that was just talk talk talk... as nothing came out of it.
My brother is now planning to go with his friends not family and said if we (mum and I) want to come we come but he is making his own plans.

To cut a long story short, I decided I don't want to go now and instead go on a cruise with my husband ALONE. We just planned this today.

When I told mum she said seeing as my brother is not showing a keen interest that his family joins him and his friends, she may come with us.

How do I go about telling her that I want it to be a holiday just me and my husband without upsetting her?
If she comes with her boyfriend, I just don't want that, because he gets annoyed and upset like a baby over silly things due to his jealousy and will cause an atmosphere on holiday and for my mum to come alone, it won't be d same for my hubby and I as I would feel I have to stay with her all the time.

Tell your mom it is a husband and wife holiday and a trip you need alone. Honestly who cares if she gets upset. Tough toodles. Your married and you need that alone time and alone trip and you do not have to answer to anybody. Just tell her the truth.

Yes quite true.
I just feel bad and don't want to upset her because she is mum!

What if she suggests to come with her boyfriend?
Its not that I don't like him but they argue over the silliest things because he gets jealous even if a man looks at her and I can't be arsed with that during a 2 week holiday!

Who cares either way, you planned that trip for your husband and yourself.
You and your husband.
If your mother tries to say both of them will go with you, you need to tell her that it is just for the two of you.
Honestly it is your holiday and your husbands holiday not your mothers. Does not matter that she is your mom or not. She will get upset or whatever tough, like I said your married and your time alone with your husband is more important.
It is called bounderies and if your mom does not understand or respect that. Let her be upset.

ENJOY YOUR HOLIDAY AND STAND UP FOR YOURSELF.

Tell her it is like a second honeymoon to get closer to each other.