You have to live your life. Both you and your husband has to understand that, neither of you you are responsible to live your lives For his mother, or his sister. Or anyone else. It is your life. You two have to agree and understand this. And that living your life how you decide, may upset people. So you two have to sit down, and talk about what YOU Two will decide, what the consequences of those actions are, and if you two can accept them.

Likely you will receive backlash for having kids first, assuming you both want children soon. And it will be his responsibility to check his mother and not allow her to be ugly towards you. Or his sister, or any of his family. Or else, if things become toxic, you two have to decide on how to handle it.

No one has the right to tell you how to live your life. If you want kids, have kids. But actions have consequences and not everyone is rational. You guys can be nice, and after deciding on what you are doing, let everyone know at board game night or family dinner. But you, as a couple, have to live YOUR life, not his sister's life, nor his mother's life.