I understand not wanting to disturb a good relationship but that is precisely what your MIL is doing. Tell her quite clearly that your family dynamics is between you and your husband. That's all she needs to know. How does your husband feel about his mother sticking her nose in? 

Though I am not a step, I had a situation with my MIL early on and made it clear via DH that things were going to change. There ensued roughly 6 months of radio silence, then a bit of awkwardness, and then we realized we had a lot in common, similar interests, and so on. She's been gone for several years now and I still miss her - she was a very good friend and the best MIL I could have asked for. I just had to define my boundaries. I suggest you do the same. 

How old are all the kid's? The reason I ask is that I was much older than my siblings (8, 10, 17 years).  No matter what, an age diffetence like ours meant I never had a close relationship with any of them. Cordial, yes, no issues there but that age difference meant we had nothing in common.  Your MIL is dillusional not to mention intrusive.

You can't care more than DH about things, you have to follow his lead regarding his oldest.  If it bothers MIL so much, have her take it up with him.   Put that conversation on to the people who should have it.  As for traveling with a baby, MIL can pound sand.  You are that child's mother and her preaching about putting the child first, need to be adults, blah blah blah, well you ARE putting your child first.   Hypocrisy at its finest.   

BM has only a portion of your DH's past. That is it. She has zero of his present and even less of his future. You are his present and his future.

Point out that there are any number of siblings in the world who have a significant age difference and unlike MIL's delusion about some forced relationship, the parents of the younger sibs have a prime duty to protect those young children from toxic situations including any toxic situation perpetrated by an older sib. Again... Period. Dot.

As for MIL's crack head like delusions of BM gaining comfort by experiencing the family dynamic that you and your DH have within  your marriage. WTF?  Really?  Why would you, DH, and even MIL GAF about what BM/XW thinks about anything. Much less what she thinks about your marriage and family.

Since you are interested in preserving a relationship with your MIL.... put the spin on this so it does not alienate her.

"MIL, DH and I have discussed how we are managing our blended family.  While we appreciate your opinion what we have landed on is that XW is nothing more than that.  She is DH's X and has no say, place, nor will she get any consideration in our marriage/family.  DH is working on his relationship with his eldest child and as our child gets a little older we will begin the process of parenting them both as siblings.  As parents of an infant, it is our primary responsibility to minimize the baby's exposure to toxic or dramatic situations and people. Regardless of who they may be. Right now SS is not in a place where he can engage positively with the baby. We will work on that. Though at no point will the baby have anything to do witih BM.  She has zero place in the life of our baby nor will she ever have a place in our family for that matter.  SS of course does. If he can engage positively in our family.  As the elder child, the primary onus is on him to engage in a positive relationship with the baby.  As parents that is the only option that DH and I will tolerate.  Thanks for your thoughts. Buh-bye."

We have similar MIL's. My MIL always has opinions on what other people should be doing, but doesn't realize that while she's entitled to her opinions, others are allowed to do what works for them without having to accommodate her. The difference with my MIL is she won't tell the person directly, she'll tell someone else, in the hope that that person will pass it along. It has ruined my relationship with my MIL because it's completely dysfunctional for an adult to try to control another adult. I make decisions that are right for me and my family...MiL doesn't have to agree with my decisions, but she needs to accept them. My bigger problem is DH doesn't know how to navigate this. Her behavior has been so normalized for him that he doesn't question that his mom would tell him what to do all the time. Hell, he even married BM because MIL told him if he didn't marry her, he'd end up miserable and alone like his bachelor uncle (instead he ended up miserable, married, and with two kids with someone he never really got along with). DH also barely notices when MIL bulldozes something we've decided upon as a family. For example, she always insists that she be part of all family events we have and always invites herself to join us or just "stops by". She also ends up meddling with BM and telling DH what to do. I honestly am at the point where I'm just going to avoid her outright. I won't keep DH from seeing her and I won't keep my DD from seeing her, but I feel drained and gaslit after every gathering with her and I just don't have space in my life for that anymore.