Wow! I just read through all seven pages of this thread! And I should have taken notes because I had so many things I wanted to comment on and now I forget most of them....
Sounds like your SIL is pretty immature to say the least. Hopefully she will improve with time. Don't know what to say about MIL....
You obviously have married into quite a family. Your dh seems to be squarely on your side though so you will be fine.
But on weddings and babies IN GENERAL (not in this specific case) - I have been to weddings ruined by idiot parents who allowed their baby or toddler to cry or otherwise make noise during the ceremony. I am always shocked and horrified when it happens. I am unable to understand how people could have different opinions on this - it is someone's WEDDING - they are vowing their lives to one another! A little respectful silence is appropriate. If you bring a child to the wedding - which I try to avoid unless the bride/groom specifically ask me to bring them - you sit in the back, on an aisle, near the door, with binky at the ready (don't know about you but my kids made noise when they nursed sometimes - either slurpy or mmmm noises which of course are darling and sweet TO ME, IN MY LIVING ROOM, not at someone else's wedding ceremony.
And with your unfortunate in-law situation not withstanding (am I using that correctly....), I do find it sad that someone would not lump his mother and sister in the group of people he loves most. I am starting to understand why after reading all the details but again in general, it is just sad. I think that being there for a sibling, on their special day, takes easy precedence over being with a spouse and child on a day which is not a special day to them. Deciding between the sibling's wedding and the child's birth - of course birth! But deciding between the sibling's wedding and... what? Hanging out in the family room? I think the SIL is obviously being rude and immature - maybe she will grow up someday. She is your child's aunt - and the future mother of his cousins. For your child's sake, (well, maybe not in your specific family....) I would do the best I could to please - all 3 of you go, dh goes to wedding while you hang outside or nearby hotel or what have you, all three go to reception if ok with bride/groom. If not then just dh go alone. I really feel the need to reiterate that this is my opinion in general for these type circumstances, not with the OP's nutty ILs.
At the end of this month, we are driving two hours (each way) to my cousin's wedding. We got the invite addressed to me and dh. Note inside (typed along with the directions) requested no kids. Fine. Harder for me to pull off - yes. Am I mad, offended, defensive that she thinks my kids are somehow less than wonderful - no. They are invited to the reception but since I'm not comfortable leaving them with the sitters the bride/groom arranged (never met them - different location than the ceremony) AND I want to take the kids to the reception to see the extended family that will be there, we are all five going but dh will take the kids out for a drive or what have you while I am at the ceremony then we'll all go to the reception. If it was friend's or coworkers or someone who didn't necessarily want/need to see my kids, I would leave them with a sitter at my house. My kids are 8, 4 and 1 but any age but younger than 3 or 4 months I think I'd feel the same - and I nursed.
Oh, remembered one final comment (I promise!) A good friend of mine was a bridesmaid at another friend's wedding when her dd2 was a few weeks old - yes, I said weeks! Out of town wedding, big fancy strapless gown, nightmare huh? But she is a lovely person and they were and still are great friends and because she loves her, she did what she had to do to participate on that special day. Her dh held the baby, I ran back and forth with her to the dressing room between photos and immediately before and after ceremony to help her nurse the baby - I'd unzip the dress while she held baby, I'd hold baby while she sat down with dress around her waist and bra off (not a lot of nursing strapless bras....) with a towel to keep the other breast from leaking on her dress! We died laughing the entire time. It was not easy (and still to this day we laugh when we look at the pix because she always says that each breast is bigger than her own head) but it was the friend's special day and my friend was the bigger person and let it be about HER, not the inconvenience/juggling it took for her to be there.
Going to stop because I'm sure this computer won't allow a post any longer than this!!!!!