Property in name of six brothers. Can they sell it without our consent

No they cannot sell the property without your sign.if you want to sell you have to get partition the property ,either mutually or through court,thereafter you can sell your portion of the land ,you can also sell the undivided share but the purchaser would not give you the market rate , he will buy less than the market value,since your boundaries are not marked,you can contact other brothers and get the partition mutually and then sell your portion

If they play fraud then they can sell the property, so we suggest that if you want things to end on amicable note then , execute a family settlement deed/ partition deed wherein all brother's define share of all brothers and after which you can sell the property and take your share.
However in case of dispute then you will have to file a partition suit in the court thereby seeking partition for the property. Alongwith this suit, file an application of temporary injunction thereby seeking stay on the property till the partition suit is decided. By obtaining this stay, your husband's brother cannot sell the property