Dear S.:
Maybe hormones..maybe a little guilt about working and not being home...and maybe an insensitive Auntie. Hard to say.
Don't suffer about the things that you cannot change. You cannot change the Auntie...and you cannot really change the working part.
I think what you can do...and this is hard... is just ignore it. Expense doesn't matter. I am sure you have appropriate things for him to wear and he is well cared for by you. YOU ARE HIS MOTHER. You have a far better blessing than she will ever have. Different people do things differently...true. Do you feel she is competing with you and judging you? Does she sense this?
Use your wisdom and feed what you want to grow. You will have more influence over a friend than an enemy...and you don't want someone you are angry with watching your child. Instead of getting offended, try this. I don't know if you can do it...but try and see what happens.
Make an appointment and ask to take her to lunch. Someplace nice.
Give her a small gift...wrapped. Maybe a photo of the baby in a frame.
Tell her, "I want to tell you something....I don't get a chance to tell you when I am dropping off the baby, but I see all of the nice things you do for him every day, and I know you love him. I am busy in the mornings and I know my son has a safe and loving place to be all day. As his mother this is so important to me and I wanted to take a moment and thank you for your help."
Try it! It will be worth the time and care your son receives. As this new baby approaches, you will need her faith in you and support more than ever.
Remember: What you feed will grow!