welbeck... "i don't know why they seem to be more involved with your niece" ...
Probably because the niece is the child of the MiLs daughter, rather than her son's child. Not saying that's right (and hopefully most GPs wouldn't show favouritism), but we all know that, often, mothers & daughters spend more time together than mothers & sons do, especially once they are married, when it often becomes apparent that the daughter's family is favoured over the son's. Therefore, it is sadly the case that many paternal GPs don't get to spend so much time with their son's family.
We know that there are many MiLs who would love to have a closer relationship with their DiLs, and grandchildren, but some DiLs don't want that. It is unfortunate, in this case, the OP would like a closer relationship with her in-laws in order that they would spend more time with her baby, and show more interest. However, it seems that her MiLs time is taken up with her own daughter and her child.
If only we could all be more sensitive towards the feelings of our in-laws, whether that's parents-in-law, or sons/daughters-in-law.