Watercolor on book pages -- I really like the idea of doing this. Would be really easy to find a random book at Goodwill on the cheap and use it like a "practice" art journal. Frame for holidays or art!
Great link for ideas like this one For sibling tattoos: Roman numerals. Could go with IV since there are four of us or each of us get the roman numeral for where we are in the sibling line up. 74 Of The Tiniest, Most Tasteful Tattoos Ever