Something happening between them once before of a sexual nature doesn't necessarily change Bella's protective "older sister" behaviour towards Jill, as it depends on the nature of what happened. A likely scenario for me would be that it was a one-time only make-out session brought about by feelings of loneliness and depression in both of them, mixed with alcohol to lower the inhibitions. Bella going through her issues with James and Jill feeling hurt by another failed date (possibly the one with Rusty), could certainly put them both in a mindset of wanting intimate closeness with someone to ease the pain, then you throw in a few bottles of wine to drown the sorrows, and there you have it.
Bella can still feel protective and want to look out for her like an older sister after this because it was a one-time mistake that doesn't change how she feels about Jill or Jill feels about her. In fact, it could make Bella even more protective of Jill because she feels as though she abused her trust and took advantage of her, hence why she'd be more determined to look out for her in order to assuage her own guilty conscience.