My late grandfather had children with two women who he didn't marry. Do they have claim to his house?

The long answer

The minor children’s mothers do not qualify as beneficiaries because they are not spouses; i.e. they were not married to your grandfather. But the two minor children of your grandfather do qualify as beneficiaries, along with you, because they are your grandfather’s direct descendants. If your grandfather did not leave a will, his RDP house would belong to the three of you equally.

Because they are minor children, their inheritance is kept by the Guardian’s Fund. This is a fund that was established to protect and manage money on behalf of those, like minor children, who are legally incapable of managing their own affairs. This fund is administered by the Master of the High Court. The minor children can claim their inheritance when they turn 18.
 
The persons looking after the minor children (their mothers in this case) can apply for maintenance or allowances from the Fund on forms J341 and J341A. These applications can be for maintenance, school and university fees, clothes, medical fees, and any other needs that can be fully motivated. The payments can be made directly to a service provider like a school or university
 
The applications must be supported by a certified copy of the applicant’s ID document, verification of the applicant’s fingerprints and any order of the court. 
 
You may want to consult the Master of the High Court, or Legal Aid, as to the best way to proceed with this. Each region has its own High Court which will have its own Guardian Fund.
 
Contact details for Legal Aid:
0800 110 110 (Monday to Friday 7am - 7pm) 
079 835 7179 (Please Call Me) 
communications2@legal-aid.co.za 

Wishing you the best,
Athalie