My sister recieved a marriage prospect which was supposedly good from all aspects and our family was looking forward for the wedding. But after a few days we revieved some pictures where the man was with his friends in several bars. His friends did drink alcohol but he didnt and the alcohol was obviously served on the table. What are we supposed to do in this situation? is an istikhara valid here? Since my sister was previously in a haram relationship my parents think that we should give the man a chance too since my sister has also done bad deeds

If he is not drinking, but was just sitting with them, this does not mean he is bad. You can ask him, and know the truth from him. If the girl wants to marry him, there is no grounds for istikhara