I like your hypothesis and I like the way you go about looking for interpretation of future events based on what the dev has already shown us in past events (from this point of view, the close-up on Sophia's shirt and bra on the floor of Sam's room could undoubtedly be a revealing element of what might happen next and lead us to think that they have some importance). However, I must point out to you (as in turn the wise
xxxorro pointed out to me when, on the AWAM thread parallel to this one, I proposed a hypothesis similar to yours) two important factors that, in my opinion, weaken your hypothesis somewhat: timing and motivation.
Sam's father may have reported to Julia that he saw her bra in Sam's room, but from the time Neil went to Sam's (and possibly saw Sophia's bra on the floor) to when Julia is about to enter the same room a considerable amount of time has passed (time in which an entire cello lesson and two drawing sessions have taken place - the first with Sophia in her panties behind the cello, the second with Sophia without panties). So why wait so long to go yell at her brother? The attitude with which Julia is about to burst into Sam's room seems to be that of a sister who is really pissed off because she has just found out something for which she holds her brother responsible, and not that of someone who has had time to reflect and blow off steam. Maybe all this time has passed because it was Neil (busy in the garage) who forgot to tell his daughter what he just saw, and so Julia has only just found out? But either way, if Sam used to masturbate by fantasizing with his sister's bra, don't you think that would be a situation that both Julia and her father would like to see stopped as soon as possible?
Then, from the distance Neil stood from that shapeless mass of clothing present on the floor of Sam's room (assuming he possibly looked in that direction), I think it would be difficult for him to distinguish a bra from any of his son's other dirty clothes (all the more so since Sam had just told him he was changing).
Therefore, I believe that Julia breaks into Sam's room for another reason, which, however, is also to be found in the past events of the game. Especially to the previous evening, when Sam, in order to pick up Ellie (who was stranded on the beach after having a fight with her shit-house boyfriend), had the kind thought of "stealing" a hoodie from her sister's closet so that she would not catch cold.
Thus, in my opinion, Julia may have just discovered that her favorite hoodie has inexplicably disappeared from her closet and (maybe having ascertained in the hallway chat that her mother knows nothing about it) now burst into Sam's room with the certainty that he took it ("Sam, are you fucking kidding me? That's so gross! How many times... do I have to tell you again that you don't have to go through my closet! Where has my favorite hoodie gone? Who did you give it to? Don't ever enter my room again!"). When she later finds out that her favorite hoodie served to warm Ellie up (and maybe it has now soaked up her scent) she will probably quickly blow off her anger and quickly go back to her room to wait for her special friend to visit, but now she's furious especially because her brother dared to enter her room when she was not there (between Julia and Sam I think there are pretty much the same rules that govern the relationship between Ellie and Dylan: Stay out of my bedroom! - in fact after almost a third of the game we practically never saw Ellie's room). At this point it would only remain to be determined how eventually Sam might be able to explain the reason for the disappearance of his sister's hoodie, before she can break into his room and find out what has so far happened with Sophia, but this does not necessarily have to happen... (it would be interesting, although it would be a further thickening of the plot).