On demise of uncle his 3 children would be absolute owner of land standing in name of deceased uncle 

2) even if they never visited they would have share in said property 

3) your father can offer to buy out his neice and nephew share in property 

1. Any cosnstrution made on the land of a p[erson is considered to be owned by the said p[erson.

2. So, the part of the house constructed by your grandmother on the land of your uncle will be considered to be owned by your said Uncle as per law.

3. Since your grandmother  did not have the title of the part of the  hopuse which was contsructed  on the land standing in the name of your Uncle, she was not the title holder of the said part of the house sha had constreucted on the land of your Uncle.

4. No body can convey or bequeath a better title than what he/she has for which your grandmother's will becomes invalid for the portion of the house she had constructed on the land of your Uncle.

5. Hiowever, your father can claim adverse possession of the said land of your Uncle and the part of the house constructed thereupon if he can establish that he has been occupying the said  portion of the house for a period of more than 12 years against the expressed wish of its title holder.

See the children of uncle has right of share on there partial of land they can file a suit for possession of same agaisnt your father for that portion , you father at this point can raise the defence of the adverse possession that he had uninterrupted possession over the property though they can contest same.

See in this scenario your father has upper hand the suit goes on for years so he can call them for settlement and can purchase the entire property by paying some amount.

Technically the children of your uncle are entitled to the land which is given by Will to them 

They obviously do not have any right over the land and house owned by your grandmother

If the children of uncle are desirous to sell the land inherited by them under the Will then might as well your father can mull to buy it instead of inviting a dispute by denying the right of the children 

Practically the children wont find buyers easily as the land is built upon and not vacant. So no buyer would be keen to take up the liability of buying a land on portion of which a house presently stands and even if any legal action is taken by any such buyer, it will take years for it to get resolved by court

So the children will ultimately turn to your father 

So its advisable to pay a lump sum amount to the children and buy peace 

Your father can then sell both the plots to some good developer or develop them in joint venture with such builder which will easily cover the cost of paying the uncle's children 

So i suggest you take a pragmatic and practical approach rather than thinking of going legally

They can't claim right now. First ask them to contribute to the maintenance. Send them a legal notice for the same to contribute.

If this property in which your father is staying was transferred to him by a testamentary deposition i.e., Will, then he becomes the absolute owner of the property so bequeathed to him.

He can file a mandatory injunction suit against them seeking to restrain them from interfering in his possession enjoyment of the property 

If your father transferred the said property on his name then your father is absoulte owner.

Your uncle childrens don't have rights to claim share in your fathers property.

Your father can challenge them in civil court

1) If property is inherited by WILL and all mutation papers are transfer on your father's name after probation of WILL than you should not worry about the plot will be sold by your cousin.

2) If taxes and bills are on your father's name than no issue if it's still on your grandma's name than a WILL should br probate. 

1. Your grandmother had bequeathed her plot to your father, so that plot has devolved on your father through testate succession. In so far as this plot is concerned your nephews and niece have no share in this property. Your grandmother could have executed a will only for her own plot.

2. However, in so far as the plot of your uncle is concerned it has devolved on his Class 1 heirs. 

1. As per the information mentioned in the present query, makes it clear that the plots were purchased by both of them jointly, and documents would also be stating the same, as I believe so.
2. There is no doubt that your uncle has given his share to his children, but at the same time, it is also need to be seen as they have never been coming or giving anything for the maitenance, but now suddenly claiming their right over the same.
3. If they happen to take the possession forcefully, then you should file a criminal complaint against them, and also ask them to go before the civil court for taking their share if any.
4. There has been provision in law as per the decisions of several court case laws as when a person is residing peacefully in a portion then other irrespective of the fact that they have some right in eh property, can be asked to pay the prior maintenance charges and may be given the option to sell their portion to the residing person.

This property does not fall in the assistant property category as the property is inherited by will  from the mother  to your father no other than him is the title holder of the property

The land belonged to your uncle and grandmother. You grandmother erected a house which spans across both the plots. Now the law is clear that the land belongs to your uncle too and hence they may claim possession. In that case the land has to be returned to them. Now you can negotiate with the relatives. If they do not respond or are adamant then you can wait and by the law of adverse possession the land which is theirs also belongs to you. This tactic can be applied.

Regards