If my drain/sewer is broken/cracked and it was caused by a third party, are they liable for the repair?

Yes, but ... You might want to contact all the parties you think are involved and advise the of the problems. You may not be the only ones with similar problems. You should then determine the actual particulars of the problem. If you need help, call a reliable inspector or a neutral contractor that can assess the situation, and recommend possible solutions. If you can work this out with the other parties, then that's good. Get these particulars in writing with all details enumerated with costs of repair (estimates are fine) and try to assign responsibility for payment.

Will you all be hiring one contractor to fix the system, or do others have their own? It can get tricky at this point, so hiring an attorney is a good idea if it becomes complicated. Once an agreement is all worked out, memorialize it in writing and as detailed as possible with milestones, dates, costs, responsible parties, etc. If you can't do this easily, then hire someone to do it. You can hire an attorney, or some one who specializes in this work and has general contractor experience. Cooperation, trust and professionalism all really matter. Hope it's a really small problem -- Good Luck!

To be fair, all you have right now is the suspicion that the work they did caused the damage. To file a successful suit, you'll need to establish the cause through the use of expert opinions. Hire a well known local plumber or two to inspect the sewer line. The news may be good or bad for your case, but at least you'll know.

dunno file a lawsuit i guess