No offense, but it's a baby shower, during covid times, in another state. I probably would not attend either. I would just chill and take it easy and not stress about stuff like this. 

I'm a little sensitive about names, so understand that as my perspective. You are concerned that DH's family will treat your older child better than your child. An understandable fear as your older child lived with them prior to you and DH being together. And realistically, they will always have a different bond with the older child. That does not mean that they won't bond with the baby, they can if you let them in.

Throughout your post you refer to "aunt". Aunt, not DH's sister, not sister in law. Aunt makes her sound likesome nebulous relative related to your older child, but not you. My youngest spent time with my brother's wife. Only when talking about her to my kids was she aunt. When speaking about her otherwise she was sister-in-law. If you want a good relationship between your child and his/her aunt, try thinking of her as family (aka sister-in-law). 

I have no idea where you are located or what the situation is there, but realize that some people may not want to come due to COVID-19 fears. That includes your sister-in-law who while she may know what your household and the in-laws have done, has no idea what the others invited have done. You may want to consider using zoom, or something similar, when you open gifts.

It sounds like she has a work issue. It doesn't sound like she is deliberately avoiding attending. 

I am so excited about your baby! Good to see you posting. I hope you are doing well and enjoying preparing for your upcoming arrival. I just got to see, but not hold due to COVID-19 and his upcoming scheduled surgery, my little five month old nephew a few weeks ago... he was so freakin Gerber-baby cute I am still cuted OUT! It was soooOOoo hard not to kiss & squeeze his little cheeks, chins, and baby-fat elbows I'm still not quite right... so much to celebtate. 

Just so excited for you!!

I think it is quite likely that your inlaws (incl aunt) will have a different relationship with your child vs the 11 yo.

Clearly they were very involved (through necessity) in raising the boy before you came on the scene.  Their bonds are almost parental and from your prior posts it's clear that your own relationship with these people has been strained at times.  They also no longer live in the same area as you and your family.. so there is also the matter of distance.  With them all being some ten plus years older than when the first child came into the world.. they also may be a bit "beyond" doing a lot of care for an infant.  So.. there are likely to be differences in the relationship.  It's very likely that they will always be closer to the older child.. but that is also a lot due to specific circumstances and not just because he is the first born..

To be honest, based on past posts... it probably will be to your benefit if they are not as overly involved in your younger child's life.  You had issues with them all overstepping etc.. and I would probably prefer to "raise my child in peace" vs dealing with overly intrusive relatives. 

Will they love your new baby?  Probably.. but the distance and relationship with you may have an impact.  

That doesn't mean that your bio child will suffer for that.. it may just mean it's a different relationship.. and perhaps easier for the child without expectations and influences from so many parties.