Although that is really nice of you to put your sister's feelings and needs before your own there is bound to be hurt feelings with your in laws. What you could do is invite them but ask them to please not bring your gift to the party ask them to give it to you later this way they can still come and all of the attention won't be on you. Explain the situation to them and ask them to respect your wishes especially where your sister is concerned and that you don't want to hurt anyone's feelings theirs or your sister's. This way you have found a happy medium because if you do not invite your husband's parents they will take it as you not wanting them there at all and this could cause strain in your marriage and your relationship with your in laws.
Also you can only invite them to your house for cake and ice cream and still ask them to give you your gift before or after your birthday celebration, I'm sure if you explain the situation they will better understand than for you to just not invite them. When you marry you become one and you have include their family because just like your are taking into consideration your sister's feelings you have to do the same about your husband's feelings also even though it's your day and it belongs to your sister and you, you have to live with your husband after the party so try to work it out before then and compromise.