Something that I did was get a fabric pattern that became our "family" letters....I made a set for myself and my diamond, and we made a set for her diamond sister....this is a fun way to spend time together and also show how much you mean to one another. Even something as small as their favorite type of candy or a card saying how glad you are to call them a sister is enough of a gift at times. Just show your love for them and for the organization in any one of a number of different ways!
Best of luck!