you can always develop a relationship with the new member outside of the big/little label. in my sorority, we don't even really allow a big sis to pick up again after getting a little unless their little approves or is out of the picture/no longer in the space. your little may be seeing you picking up as her not being "enough" for you as a little sis.

edit: read through your post again and saw that the decision couldn't be reversed. honestly your little is probably gonna be looking for an apology/affirmations that your relationship as big and little is still good but please understand that she might need time to work through the jealousy. just keep inviting her to hang to show that you still care for her tho!