Your first post mentions you’re feeling “left out”, which is understandable, but try to think of it as nothing personal. To me, it sounds like she just wants some peace and quiet and is getting overwhelmed by the continual contact and invites from you and your friend group. Some people would rather not say that outright, but just ignore and not engage in the hope that you’ll get the message.
We’re all different. When I got out of hospital with one of my DC who was quite ill at birth, I was exhausted and stressed and just wanted to be alone. However, a colleague knocked at my door the day after we got back. I ignored her and didn’t answer. From my upstairs window, I could see she had a card in her hand so assumed she’d just put it through the letterbox and go. But she didn’t. She went - but she came back twice the next day still feeling the need to get me to open the door so she could hand me the card rather than put it through the letterbox! By this stage, I was quite pissed off with her. She was a lovely woman, nothing nasty or strange about her, but her ideas of what was wanted and normal were very different from mine. I certainly wouldn’t have sent her a photo of my baby btw, because it would have ended up on a billion Facebook pages. Again, just her doing what she thinks is fine and normal, but I didn’t want photos of my children online.
Four and a half months is still young. I found I really started going out more at 6+ months as baby sat up more, fed less, etc. It’s hardly weird, especially as she’s been seen out with others.
Keep in touch with her brother and ask after her when you see him, but back off and leave her alone for a bit.