You have sale deed, your ownrhsip is intact. If no stay from court in on going case, his objection has no value. You go ahead with construction and if he obstract, complain to police.

Also check from seller, under what capacity he is claiming ownerhsip.

Is there any stay order passed by court restraining seller from selling the property 

if so ask seller to refund your money as title is not clear and marketable 

if no stay orders are passed ask seller to cancel sale deed and refund your money with interest as he failed to disclose pending partition suit 

You dont worry about their family feuds.

You have to rely upon the registered title deed on your name.

If he has got any claim then he may have to fight and drag his father in law to court, he cannot obtain any injunction against you nor he can object to your construction.

You can  challenge his case properly with the help of a skilled advocate if your current advocate is not able to handle this effectively

Dear Sir, 

You are suggested to submit an application under order 1 rule 10 of the CPC in the ongoing suit that the you should be impleaded as interested party in the case and after this application is allowed establish your claim that you are the rightful owner and the partition suit is infructuous now. 

Now you can go as per partition suit and apply for main owners share that share will be yours. 

You need to approach the civil court for declaration of the said right. But of you don't have possession then you need to wait till decision of that partition suit

You first collect land records from the office of Tehsildar who owns the property and how many legal heirs on the record?

If partition suit is pending before the Court of law then your chances of buying the property is less and loosing is more.

Better get all records of land searched and verified about the ownership and heirship due process of law done by due diligence process. 

- As per law, a Hindu cannot do second marriage without getting divorce from his first wife , then the second wife is not a legal wife to him , and she having no right to claim over the property of him.

- Further , if the property which you have purchased , is a self acquired property of that seller , then he was having his right to sell it , and none having right over the same. 

- But, if the property was ancestral , then the children of first & second illegal wife can claim right over the property .

- Since, there is already case filed by the sellers daughter , then you can lodge a complaint against the seller for the offence of breach of trust and cheating . 

- Further , you can file a suit for Permanent Injunction against them for restraining from creating any hindrance in your construction work .

Without seeing the plaint it us very difficult to advise on merit of the suit.

However refer to your sale deed. If there's any indemnity clause then you better get compensation from your seller and leave this property. 

Otherwise you will have to wait till the partition suit ends. 

1. When the matter is under subjudice, any transaction concluded in respect of the land which is under litigation will not be a full proof legal transaction and does not confer clear title on the purchaser.

2.  File a cheating case against the seller, who sold the property to you, when it's under litigation and the seller was aware that there is a ongoing case on the property.

3.  Bring an Injunction Order from the competent court, to prevent the son-in-law of the seller from interfering and trespassing into your land.

If the property is a self acquired one by the person then nobody can raise any objections on the sale of this property and least of all the son in law.

If it is an ancestral property then the children may raise objections.

You didn't receive any notice from court so there is no stay over the property, if you have property documents than continue construction and if someone comes to stop you than file a police complaint 

We suggest you should consult a local lawyer who can examine facts of the case based on the documents and facts.. 

It would be proportional 

You can establish your rights by claiming that seller shows document to prove that land is seld acquired by him and his children cannot claim any share from property.