Ben Torah, the only Torah “source” that I can recall is the story of Rachel and Leah where the “tzaddik hador” Lavan, justified his tricking Yaakov into marrying Leah by saying it is not done in their circles that the younger sister marry before the elder. If you want to use Lavan as a source…
That being said – the only idea I can see is that the younger sibling should ask the elder one for her bracha, out of respect for that sibling (based on the halachos of kibud Av, which extends to one’s older siblings).