Hesitatingly entering the fray ... my beloved younger sister was adopted when I was five years old. She was a baby but had been horribly neglected in the private 'home' where she'd been placed after birth (this was the 1940s - and that sort of thing did, alas, go on). She was in a sad condition - malnourished, skin problems, nervy and jumpty when she came to live with us but, with the help of a wonderful GP, was nursed back to health.
These days, my parents wouldn't have got to first base in the adoption process, both being serious depressives, but to give them their due, with my tiny sister, they pulled out all the stops. I was jealous as hell and this didn't really abate until my teens, because little sister got SO much attention from my parents, relatives, friends - and quite right too, in an effort to make up for her truly horrible experiences in her first months of life.
Perhaps the OP's nephew has suffered awful things before being removed from whatever situation he was in and put up for adoption and the grandparents are going all out to make up for this as much as they can. He's four years old and who knows what ghastly memories he has and possibly also long term problems - the fact that he still needs nappies would indicate this, poor little boy.
The grandparents are obviously making a HUGE contribution towards caring for the little boy - I find this entirely laudable. But I am so sorry the OP feels her baby is being left out - sounds like I felt when everyone was concentrating on my new baby sister when I was five.
But the little adopted boy has a huge raft of problems and needs. The OP's baby has two loving parents, other loving relatives and friends and really, I'm sure the grandparents don't mean to seem indifferent to her child but it's just that at present there's a huge effort of help the little adoptive boy.
As for the sister in law, she sounds pretty awful but I agree with Randomnessesses up-thread who posits that she is possibly very jealous that you conceived, have a partner ... and she's obviously pissed off that you got her brother! What she's said to you is nasty and juvenile but be the better person and rise above it.
I think things will settle down. Your inlaws still have your baby with them once a week, which as others have pointed out, is more than many GPs do - and I wouldn't worry about lack of presents, taking nappies, food etc. All pretty unimportant in the great scheme of things. I hope you and your partner will be good aunty and uncle to the little boy - he's probably had a very sad start in life and deserves a lot of love and support, which I suspect is what motivates your inlaws.