The validity of the contract would have been mentioned in the contract agreement itself.  Based on the contents of contract and guarantor deeds, the validity can be determined.

Hello,

1. One needs to go through the contract to form the right opinion. If the condition not to take up anotber employment in India is limited to a specific period of time, it may be valid.
2. Change of job is your right and unfettered in as far as no promises are breached.
3. As stated earlier one needs to see the draft contract to form an accurate opinion.

terms of contract are binding upon you 

if you commit breach of terms of contract company would seek to enforce the guarantee 

Contract of service can easily be termed as ' when one person has agreed to employ another person as an employee, and the other person has agreed to the terms of employment and of an employee, then the party have said to be entered in Contract of Service

It is a common practice amongst employers in India to have a provision incorporated into the employment contracts that in some fashion restricts the employees from pursuing activities contrary to the business interests of the employer.

It is well-settled that a contract which is in restraint of trade cannot be enforced unless it is reasonable as between parties and it is consistent with the interest of the public. These two principles are the sine qua non for a contract which is in restraint of trade.

Section 27 of Indian contract Act, 1972 states that, "Agreement in restraint of trade, void – Every agreement by which any one is restrained from exercising a lawful profession, trade or business of any kind, is to that extent void.

In Niranjan Shankar Golikari v. Century Spinning and Manufacturing Company Ltd.,
(1967 AIR 1098), the Supreme Court of India considered a case of an employee who had
accepted the employment offer and signed a contract for a period of five years. The said
contract contained a stipulation that during the period of employment, the employee shall
not engage in any other business. The Contract also contained a stipulation that during the
continuation of employment as well as thereafter the employee shall keep confidential, the
information, instruments and documents of the employer that might come to his
knowledge. In the said case the employee after receiving training and acquiring certain
confidential information, initially remained absent and then tendered resignation, as he had
obtained another employment. The employer did not accept the resignation and took
recourse against the employee by seeking injunctive relief and damages. The employee
challenged the validity of non-compete and confidentiality clauses, contending that the
same constituted a restraint on trade. Dealing with the said contention, the Supreme Court
held as under: “The rule now is that restraints whether general or partial may be good if
they are reasonable. A restraint upon freedom of contract must be shown to be reasonably
necessary for the purpose of freedom of trade. A restraint reasonably necessary for the
protection of the covenantee must prevail unless some specific ground of public policy can
be clearly established against it

Therefore you may take proper legal action in case your employer is initiating any legal action on you tendering the resignation.

First of all, without seeing the contract, I cannot say anything.

The Terms and conditions are required for deciding the validity.

The purpose of the contract may be :

If anything goes wrong on your side, your brother will be held liable for your acts. 

An employer incurring expenses to send an employee abroad on any assignment is entitled to take assurance from employee that he will not leave the assignment midway. After completion of assignment the employee is free bird, he can take any new employment without consent/permission from ex employer.  Any bond/guarantee that restricts the employee after completion his assignment is invalid, illegal and not enforceable.

If you sign you will be binded bhe same. Need to go through it fully

Dear Client,

Every agreement of which the object or consideration is unlawful is void.

Thank You.

There is an adage that no one can serve two masters at the same time. In your case, you have to work whole time for your UK employer and cannot do any part-time job for any other employer during the contract period. This restriction is perfectly legal.