well, living with in-laws are like that want. after all, they are not yr blood kins but since you have not choice but to stay there, have to give and take. its good that your mother in law willing to look after yr baby and ask you to take dinner.
How old is yr gal? How old are his parents? How old is yr brother in law's son?
i am now staying with my in-laws and have been living with them for 5 yrs and the clashes get more when my son is born. Now my son is coming to 9 mths and still, clashes are there.
Are you able to buy yr own house and move out? I would always prefer to stay ourselves but now still waiting for our flat in 2014... got to endure till then. Another way is put yr gal in infant care, will have professional to sing, read story to them. i put my son in Infant care since he was 4 mths, the only con is, he got sick easily, other than that, the care takers are good.
as for no poo for 2 days are common. give warm water and massage his tummy will do. Is yr gal on BF or formula?