Physio definitely would've spotted it.I’ve mentioned my SIL before she is a looney
he defo sounded like twin 2 and was definitely behind if you had have asked me 2 months ago but if he isn’t expected to be walking then I assume he has caught up. I have been worrying about it because she is relentless. In fairness he is also a ridiculously good sleeper and very chilled so isn’t exactly getting a lot of practice
Tell her to mind her own unless she knows what she's on about she doesn't need to be throwing diagnosis' about.
Boys can be notoriously lazy, my 10yo didn't sit unaided till he was 9months.
Hell get there just keep encouraging him