What evidence COs want in different cases seems to vary, so it isn't possible to be definite as to exactly when you will have enough, for your particular CO. But do keep on fighting. As you are a genuine couple, and since, as time goes by, you are going to be able to give more and more and more evidence that you are together, there will be less and less room for them to doubt you.
Do you think it will help if we will have also some statement from my parent's in law?
Yes, it could be a good idea, and perhaps from someone who knows their family also - neighbours who know you are there, or perhaps someone professional, like the local doctor or police officer. When my son was getting Residence because he had married his NZ wife, the CO asked for such letters from people on both sides of the family. Let the people write in their own words - this need not be formal. It is important that the CO can see that the people have come to know you gradually, and met you on different occasions, and, naturally, are perhaps aware of different things about you, depending on what conversations and interests you happen to have shared. It mustn't look as if all parties have conspired to put a story together!