Hi Kish!
I can already tell by your question that you are a sweet person...so no matter how you invite them I don't think they will be offended. Your heart is in the right place!
"Invite" is the key word here. You are inviting them to share an experience with you. I would tell them that the membership has been so amazing for you that you really want to share this with them and create new family memories together.
If they shy away and cite money as a reason not to go that's when you mention that your membership provides so much for you that there is no way you can use it all...that those benefits will go to waste if they don't help you take advantage of them! The money has already been spent and now you need help making the most of it. And if he has any questions, send him here where some of our moms are DVC members and can help him out.
My family had a good friend send us to take advantage of her DVC points last year. At first I said no because I thought we were taking something away from her. I thought she really felt sorry for us, and well, I have that silly thing called pride sometimes. But she explained that she had already been to Disney earlier in the year, and that she had no plans to use those points anytime soon, so they would be wasted! I might have silly pride, but can't stand anything to be wasted! So we took her up on it and had an amazing time.
Kish, I know you will have an amazing time with your Brother's family making Disney World memories together! Take care, and keep on being a great person & sister!