1. Anybody in possession of a land for more than 12 years against the expressed wish of its actual owner can claim adverse possession of the said land.
2. In the instant case, your relative is not in possession of the said agricultural land but was firming the same with your consent for which it can not be termed as adverse possession.
3. Lodge a police complaint against him for trespassing in to your mother's land against your wish
4. File a declaratory suit praying for a declaration that the said land belongs to your mother and she is within her rights to restrict anybody's entering in to her said land.
5. Simultaneously, file an application u/o 39 r 1 & 2 praying for an order restraining your relative to enter in to your land till the suit is disposed off.