When History Repeats.....

I am not sure whether to interfere in this matter or not.
Part of me suggests that I should take active part in solving this problem, since I have first hand experience similar to this mess. I feel, it is best to interfere and help before a disaster happens.
However, the other part of me warns that it is not my business, and I should better stay out of it.
So, I am here looking for your expert opinions on this matter.

This is about my BIL and co-sis.
They are younger to us, and married only for ~4 years. They are yet to become parents, and that itself a problem already in their life.
It was an arranged marriage, and co-sis is MIL's cousin's DD.

The issue:
During the marriage arrangements, there was an open talk about dowry.
Co-sis was previously engaged to another guy from their native; thus her parents started to build a house in that place in order to gift it as a dowry around her marriage. However, their engagement was called off due to some issues; thus the construction was also stopped.
After a year, BIL came forward to marry her. Since BIL has no business in that city where the house was partly build, they requested a house in the city where everyone lives - which is a metro city.
But it is very costly to own a house here, co-sis's parents rejected this idea right away and opted for alternate arrangements. During the talks, PILs were not open and most of the demands were put on hold or awaited for a later discussion it seems. There were many common family members involved in this marriage discussion.
Since they are relatives anyway, they went on to marry.

Immediately after marriage, BIL suggested co-sis's family to complete the house in the other city, which twas partly build before. Upon completion, he insisted they write off the house to co-sis.
A few weeks after, BIL suggested moving to co-sis's parents' house on the pretext of expired rental agreement of his (then)house. And eventually he said he is uncomfortable to stay with his in laws. Thus PILs suggested co-sis's parents to move out to their native, where they have build a house for co-sis as dowry.
To avoid conflict in the new couple's life, co-sis's parents moved out. But her dad became ill, and needed a bypass surgery immediately. None of their children were living in that city; thus it became difficult for everyone to attend to that emergency.
As suggested by many, they moved back to their original home and suggested BIL& co-sis can have separate cooking arrangements if their stay disturbs them.
But BIL was unhappy and moved out of that house and didn't take any effort to welcome co-sis to live with him.
Due to this, her parents rented off a house for the couple, and paid advance money for 1 yr.
BIL & co-sis happily lived in that house, until the rental agreement expired.

Now that BIL requested her to pack her stuff and go back to her parents' place as he doesn't afford to rent another house right now.
Co-sis and family suggested BIL to join them in their house, as it is very big. But BIL says, unless they write off this house to his or his wife's name, he can't live there.
Also, he temporarily suggests co-sis's parents to move out of this house, so that he can join his wife. This time co-sis & family refused this, and did not enter into any kind of talking in this regard.

PILs and BIL are unhappy that co-sis is not working, and completely depend on him for everything. They criticize her cooking and house keeping skills, as though she is a burden on him. Her infertility (not sure who has problems) is highly bashed.
They wouldn't have married her unless there was no hope about this city house. PILs & BIL considered the house in the native as a bonus.
Now that, they regret their decision to marry a stay at home wife for such a simple dowry, that too inaccessible as and when they need it.
Besides, they are afraid of the upcoming expenses such as fertility treatments, child birth, school etc with one income, and think a lot about how to correct their mistake (of marrying co-sis).

Besides, they criticize co-sis parents for not letting the new couple live happily by giving their city house.
Now that they urge co-sis to sell her dowry house, so that they can have some money in FD - which could be used as key money whenever they are in need of new rental house. Also, PILs hope co-sis could cover up her own expenses using that bank interest. With this, they basically hint them that BIL is not ready to treat co-sis's fertility issues (if any). They haven't considered any treatment process for now.

Co-sis & family thinks PILs are after money. They are afraid to sell the only asset they have in co-sis name and give the money to BIL. Looks like BIL will give the money to his dad, and the rest is known to all.
Co-sis tries for a job, but with this gap, she may not secure a good post. Her parents' city house may be shared among her other siblings too. So, they can't have any hope on this house.

As of now, BIL and co-sis are separated and not in touch with each other over a month. BIL threaten D word, and say he plans to become as a PG in some men's accommodation near his work place. In that case, he threatens, he will never live with co-sis again.
He visits PIL every other day, and they all discuss on this matter very deeply. Apparently, MIL has invited co-sis's parents for a meeting in this regard, but they have turned down this invitation.
They say, they are ready to discuss these matters before a 3 person/probably a family member. To which PILs said NO, citing MIL's health problems.

Co-sis speak to me almost daily in this regard, and her stand is very clear as I said above.
BIL & my H discuss this often, and my H suggests that I should stay away from this matter.

I feel like giving co-sis some moral support and suggestions from my experience and that of others like you all.
But I also don't wanna spoil the soup with my interference. What you all say????