K.F.
This is a bit late since Mother's Day has past....but I know kids are resilient and very accepting - and still don't understand all the complexities of relationships, no matter how bright or how well-explained. You can just answer questions as they come honestly, and say the family is very excited about her new 'aunt.' She will probably be happy to get a new aunt as well and may overlook the specifics...if adults talk about stuff in front of her and she has questions, then just explain it in a kid-friendly way and she will accept it....no need to be too detailed, simple answers usually suffice. If she keeps asking, that's good...she will eventually probably get bored asking questions and just go off to play...funny story - my step-kids take for granted I'm their step-mom, but still kind of don't understand the 'family tree.' my daughter was asked to draw one, and she put her bio-grandma and step-granddad as MY parents' parents and then put my DH's ex clumped in with me and my DH, and her half-sister and her and her brother all together under all three of us(even though I have no relation to her bio-mom's DD). I thought it was funny, but didn't correct her because I didn't feel it was necessary - she loves my parents and her bio-grandparents both, so that's what's important any way.