They probably didn´t even know what gay was about and thought, what a polite, nice looking young man! 
I don´t know how June and Nevill perceived Brett! Perhaps they liked him and disliked Julie (we know that), and that made Julie furiously jealous of Brett.
I have never seen it mentioned anywhere that they disliked Brett.
To be honest,,I think that the fact that Jeremy was tagging on to Brett Collins was that his relationship with Julie was wearing thin,,so Collins came in handy as an excuse to get out of seeing her less often.
June was probably relieved to see a different face than JM's,,so would have welcomed him to the fold.Whether either parent knew Collins was gay,,I wouldn't know. They'd have soon heard from all the gossipers,,RWB especially.I'm surprised he didn't tell Collins to wear a bell round his neck,,he was so incensed.
JM just came in handy at weekends.

Well you don't look at the mantelpiece when you're poking the fire,do you ?