DSS affecting DD and DS

[inabit]

This may come out all wrong but going for it anyway as it's driving me mad! DSS is 15 and has a total lack of respect for me and my DP.
His behaviour and attitude is spreading to my DD (6) and DS (3) so now their acting like 15 year olds.
DP shrugs off his behaviour as 'hormones' yet wants to punish the younger ones as their 'just naughty'
Sometimes wish I'd not taken him on

First and foremost -You need a united front.You need to tackle this as a team and to be firm with dss.

He is old enough to know his behaviour is abusive and also detrimental to the whole family unit.

Do you have any support? has dss's behaviour crept into his school environment?

I know he realises what he is causing its as though he doesn't care.

Not much support from extended family as due to his difficult upbringing whilst he was with his mum its as though he can't do anything wrong.

As for school his behaviour was great until January this year when it has got worse . We've been called into school only last month because of his attitude and disrespect.