You are a good mum; you are an incredible person to withstand so much, please remember that.
My dad died when I was pregnant with my dd, it is nothing like losing a child, I know, I totally understand that. It did make the pregnancy difficult. You have my total sympathy for this terrible experience.
I do hope you will find help to process your grief. Experiencing grief when there is a new baby around can be very hard. The first few years of my dd's life we spent a lot of time around my mum, at her home, away from our own. It seemed like almost every weekend, and she was grieving of course. It took a long time for her to overcome that grief, to some degree. But my dd, as a baby, did not think it unusual that we spent so much time with her grandma etc. In the same way your dd2 will obviously hear about your dd1 and grow up knowing about her. I guess what I am saying in a jumbled way is that parenting your new lovely dd and grieving your lost lovely first dd will happen at the same time and the sorrow and joy can mingle and to some degree you will find (I hope) the peace amid it all.
I am not sure about the right terminology but there does seem to be some process of moving through the grief, over time, if does happen to some degree, I've heard, even for those who have lost a child. If you can access any help, someone to talk to, please do.
Cruse bereavement care is an organisation that has some sort of support
www.cruse.org.uk/
There is a help line and various things on line, and sometimes in some areas too.
I hope and pray your dd2's Christening will be a time of joy. I am sure sadness too, but also some joy.
Please discuss this horrible issue with our SIL before the Christening day, as soon as you feel able.
You may wish to consider whether it is right to ask her not to come to the Christening, if you or your husband do not wish her to be there. I know she is your child's aunt, so this may be hard for her or your husband. It may be she apologises and you feel able to include her in he day.
Either way, please do discuss this before the day.
Do not allow her callous attitude and this specific comment to detract from your, your husband and your dd2's day.
Thinking of you.