1. He is only going to be coming to visit us and for sightseeing. We bought a house so they will be a part of the house warming ceremony.
He will be in Canada for a maximum of 25 days.
For all applicants, I recommend you prepare a day-by-day itinerary with expense estimates for sightseeing, meeting family/friends, etc. Do not make the house celebrations as the only reason to visit.
Also, for your brother-in-law, he should keep his visit to 2-3 weeks max, preferably around some national holidays to explain why he can take time off work for longer.
2. He hasn't travelled anywhere else, always been in India.
Any possibility that he has to travel abroad because of his employment?
3. I can see if he can show any of those things.
Can his manager put a note saying he has to be back by this day for so and so project deadline? Would that suffice?
He must also include 2-3 bits of evidence that he is indeed part of that project.
4. He doesn't have any major reasons except his work and parents being in India. Is there anything else that can prove a reason to go back to India?
IMO, your brother-in-law does not have a very solid application. You would need to work really hard to think/include other reasons for him to return his home country. That's because he is young, recently employed, not married, no other movable/immovable assets, no travel history.
Also, all three applicants applying together would reduce their family ties to their home country to nil. Each applicant would need to demonstrate his/her individual and collective ties/reasons to return home (with evidence)