I really think that you need to think long and hard @Yellink about marrying into this family. If your partner had agreed with you and said to his sister and Mum (as politely as he wanted to)
"Fuck Off DSister and DM, you had better get used to the fact very quickly that we are our own family unit, and will make any decisions about what we do about anything ourselves. We will not be dictated to, so if you want to have a harmonias relationship with us, you will never try to lay down the law with us again"
But unfortunately OP your partner didn't say that did he? He doesn't care when the Tree goes up, so he should have sided with you immediately, but instead he wanted to do what he thought would give him a more peaceful and easier time. So he told you to leave it until Sunday.
Do you hope to have children with this man OP? Because if you do, forget about whether you want to breastfeed or bottle feed your baby, forget about you and your partner deciding on your child's name, or what sort of nappies they wear, or whether you will feed your baby on demand or have set feeding times (but within reason do please feed them on demand!)
If you have a dining room and you and your partner decide you want it to look quite dramatic, so it's over all colour scheme will be black, don't expect his Mum and his sister to like that idea at all, so it would be another "no" from them, with your partner agreeing because he wants an easy life. So if you insist on staying with your partner, I think he either needs to start showing you right away that he is going to be standing up to them, or, you need to show him that siding with his family does not give him the easy and quiet life that he was hoping for.
I can promise you OP that if you stay with your partner without standing your ground, you will in the future bitterly regret it. I can't tell you whether that will happen next year, in 5 years time, or in 15 years time, but you will very sadly end up regretting it, which means that you will end up being very unhappy. So please stand your ground now, and don't make excuses to his family about why you are putting the tree up today. Because you want to, is absolutely the best answer.