Nora ~ I am so sorry for this difficult time too, I don't think you were overreacting at all. With the death of my daughter I have realized that people just don't understand [:(]. I think what everyone has said is great and feel free to "talk" here anytime. I hope that you and DH work together on this, that is the most important.
Just wanted to add, I understand the feeling of not wanting to be around babies, pregnant women, etc. For me this was after the loss of my daughter.I have a nephew who was due on the same day as my daughter and seeing him is very difficult. Being with family & friends was very difficult, I always felt awkward, thinking everyone always had pity on us. I have tried hard to put that aside and focus on them caring about us, it is still hard though. I definitely don't think it is right about your MIL inviting anyone over, but maybe you could slowly get involved (a short get together at maybe a park) with you SIL and her kids (or someone else with child your son's age) to help with this. For me taking baby steps has really helped, don't push yourself. In the beginning I could even go to a get-togther without being almost physically ill, then slowly I could stay more each time. Lastly, definitely only surround yourself with positive influences or you and your family though.
Really hoping the best for you, take care.
Tyler - 4 (36 wks, PIH (just told by peri it was mild PE))
Angel baby - 1/20/03 (11 wks)
Faith Kristine - 1/5/04-1/30/04 (30 weeks due to severe PE, passed due to NEC & Sepsis(preemie complications))
trying for #2 to share our lives with since 9/02