Daniel,
During a prior marriage, while she had slept with many other men (my co-workers, close friends, rivals, her ex-husband to strangers), my than wife always said that she would never be sexual with one of my very close friends (like a younger brother to me) specificity because of what you described in your posted #439 about her baby sat him in the past.
Although the deeper down the cuckold rabbit whole we ventured, the much more adventurous she became and was open to areas in which she had previously considered taboo. This in turn lead to the next time this specific friend made his desires very clear to her, she surprisingly accepted his advances and reciprocated. Was a creative exchange between them.
The next time we were all together she made it crystal clear to me in front of him that he was going to be fucking her any way he wanted that night. After the first session that same night, she told him in front of me how good the sex with him and that he was able to bring her to orgasm quicker through intercourse than I could. She made sure that the experience was a bit humbling and humiliating for me while playing up to his ego at the same time.
So I would say anything is possible if you wanted Jen and your brother to explore a sexual option.