You know the sister better. There might be undercurrents - the relationships between sisters differ SO much from one family to the next. She might be looking out for you; she might be undermining you; she might be undermining him; she might be provoking you; guessing what her motivation is is extremely hard, since a lot depends on her own relationship status - if she is single she might secretly begrudge you your coupled up status and try to ruin the fun for you under the guise of a protector.

To her report: she said that he is adding lots of girls. You said that he always had lots of female friends. And that you have lots of male friends. Do you realize that people's attention spans are fairly limited, as are other resources; had he been into another woman currently, there would have been a spike in his exchanges with one particular facebook contact, but your sister reported his having been adding lots of women, which does not make it appear that he is into somebody in particular, plus, is not out of character for him, and, another plus, the reverse is not out of character for you (wrt male friends). So what was the NEWS in your sis' intelligence? nothing. No news. And since she reported something that was not news, it makes me wonder if she might be having ulterior motives. You never know. I am an only child but I have observed such a tremendous variation in how sibling relationships unfold around me that nothing would surprise me. Plus, don't the most compelling biblical stories talk about less than amicable sibling relationships?