My sons behaviour has suddenly changed!!

Ok so lately my sister and her son have been staying with me since the 1st of July (she was supposed to move somewhere but the previous tenant refuses to move) but since they’ve been here I’ve noticed a huge change in my sons behaviour (keep in mind he hasn’t been in his own space for almost a month i.e his room) ., he is starting not to listen more I’m guessing because his cousin doesn’t listen that he thinks it’s ok ., he’s also gotten more aggressive I find myself having to tell him to stop hitting when I never had to before., he refuses to be punished , when I put him for a time out he just gets up and does his own thing now when he wouldn’t move until I came to get him from time out , no matter how many times I try to enforce it . Not going to bed on time when I’ve had him going to bed since 7:30-8pm for a year and a half ., now he’s asleep by 10-10:30 ., needless to say I’m getting very frustrated and I’m not sure what to do ., i feel like his behaviour comes from his cousin who has no discipline (my sister is very lax 🙄) my son got the hitting from him & the worst part is he is 6 months younger ??! I’m starting to get tired of having to constantly repeat everything to him ., I’m just tired overall 11 weeks pregnant I hope this behaviour doesn’t continue 😧😵☠️