Hi Sonia,
MashaAllah you’re very lucky that you have such wonderful in-laws that you don’t want to move away from them. Most of the stories here in the Life forum are the opposite where the girls have dreadful in-laws and moving out is not an option for them.
It’s a tricky situation. Has your husband provided any other reasons besides “it’s the right time?” I don’t think it’s a good idea to tell the parents that he’s moving out RIGHT AFTER they come back from Hajj. His parents are going to come home exhausted from Hajj and they’re going to come back looking forward to reunite with family and feel that spirit of togetherness…and then he plans to hit them with this news??? He’s not using his head.
If he wants to move out, then he should have this discussion at a more suitable time. I suggest that you urge your husband to have this discussion after your SIL’s wedding. A wedding should be a happy occasion for the bride and her family with fond memories to look back upon. Your SIL should not have to deal with family drama and grudges during the time of her wedding. So explain this to your husband and urge him to wait until after the wedding. Meanwhile, continue talking to him and urge him to explain his reasons for wanting to move. If you are so set upon wanting to stay with his family, then explain the benefits of living with a joint family to him. Explain to him that you don’t want his mother to think that YOU secretly advised him to move out and that he should not make it appear to them as if you had any role in his decision (that would be unfair).