1. Since this property as bought on three people name, it becomes the joint property of all the three i.e., yourself, your brother and yor mother.
After partition all of you have been allotted with 19 acres of property each.
This indicates that you are the absolute owner and have marketable title to your 19 acres of land alone and this positions equally applies to other two too.
Thus upon yr mother's intestate death, her share of 19 acres of land shall devolve equally on all her legal heirs which consists yo and all yor siblings.
Therefore yor sisters are entitled to 1/4th share out of your mother's share of 19 acres of land which have been divided by you and yor brother alone.
You may pay them an amount in lieu as compensation for the landed property to the extent of their entitlement and gt their signatures in the sale deed.
Your children or your wife do not have any rights in the properties hence the demand for their signatures in the sale deed is unnecessary and unlawful.
2. Legally you cannot avoid the signatures of your sisters in the proposed sale deed.