Everything @MsDogLady said.
I wouldn’t have been in the ‘every woman is a potential Other Woman’ camp suggested above, but for the OP’s update.
This woman has settled into OP’s home and is playing queen bee and sidelining OP, with the two men complicit in that. I may be reaching here, but is she presenting herself as the ‘cool girl’? Fun, outgoing, unencumbered by kids?
DH is prioritising her/them because he wants to, not because he’s too stupid to read social signals - and he’ll have been flattered by the shirt stunt, which was a definite power move on her part. There may be nothing going on between them, but they’re both enjoying the dynamic and getting something from it.
The whole thing is blatantly disrespectful and hurtful to OP, but I can see how it’s difficult to broach with him because there is so little tangible ‘evidence’ for OP’s feelings, and he could easily choose to get defensive, putting even more distance between her and the three of them.
The one fact he can’t deny is that he’s spending more time and money on these people than his own wife and child, and it’s hardly like they’re visiting for a few days. They’re living here for 5 months rent free, so surely they can sort out their own activities? I’d insist that he makes an effort to redress the balance, and schedules plenty of things for you to do exclusively as a family for the remainder of their time here, especially on weekends.