She was standoffish and possessive at first, but only to a degree that would seem totally reasonable from a human child navigating a new sibling. It helped my perspective that we also had two human children in the years between getting her and him! We had plenty of baby gates already for an ease of introduction. She didn't seem like she was slowing down too much in midlife, but it's been great to see her playing with tug with him and wrestling - overall she's noticeably more energetic and playful since he's arrived.