When is a lease a constructive sale? 49 years?

My husband and 6 other grandchildren, all British citizens, inherited a 1,000 acre estate in Uganda from their grandfather. The title deed filed with the registrar names them as joint tenants. Currently 2 of the 7 co-owners live on the estate; one has recently become a Ugandan citizen by applying for dual citizenship and the other has an application in process. The other 5 co-owners reside in England with no ties to Uganda beyond this inherited property. The co-owner who has dual citizenship intends to demise ~100 acres of what is currently farmland to residential lessors by creating long term leases (49 years), to be offered to “promising young Ugandan citizens and foreign nationals” of her choosing, at a stipulated price which is very likely below the open market price for this land. At least 2 of the nonresident co-owners wish to sell their interests, and would like to do so by preferentially receiving the proceeds of any income generated by the land, including any lease payments. As I understand the law of co-ownership, a joint tenant cannot sell the land but may be able to LEASE it to third parties without the consent or knowledge of other co-owners? Is there anything the other co-owners can do to prevent this? Further, it has recently been disclosed that a few years ago one co-owner emailed his sister (the prospective lessor mentioned above) and offered her his share. She agreed and transferred £20,000 pounds to her brother. The transfer was not recorded with the registrar of land titles. Presumably this creates a 1/7th tenancy in common interest which the sister now holds in addition to her joint tenancy with the 5 other siblings and cousins? These 5 are objecting to this on the grounds that the transfer was at below the market value and the opportunity to participate was not offered to them. Do they have any recourse?