Yanbu.
MIL should not be co sleeping with a young baby if she doesn't wake up when the baby cries. Of course not. She could have rolled onto the baby and suffocated her.
Also she shouldn't be watching baby overnight if she doesn't wake up when she cries. Absolutely bizarre earlier replies to this. Yes babies do cry, but as parents we shouldn't choose to put our babies in situations where no one will respond to their cries. 10 minutes is a long time to a tiny baby who is hungry or thirsty or scared or in pain, and doesn't yet understand that her carer may be asleep/ downstairs and will come later.
Also I don't think MIL should have co slept without asking you. I co sleep with our baby and love it- but we researched how to do it more safely. Just falling asleep unplanned with baby next to you carries many more risks.
I wouldn't let DH sleep next to our newborn baby for example. Mums/ the primary caregiver, especially breastfeeding mums, have more awareness of baby next to them, and will wake up more easily, and are more likely to be aware of baby when they move the covers and move around in bed.
I don't understand why MIL is pushing to have your baby overnight at 6 weeks though. Completely bizarre (unless you are desperate for some sleep and she wanted to help you). At this age baby is safest and happiest with their primary caregiver (usually mum) overnight.
I would veto any further overnights and specify no co sleeping for daytime naps, and encourage MIL to look after her in ways you are comfortable with, e.g. in the daytime. Don't feel pressured into having MIL care for your baby in ways you don't feel comfortable with- your baby needs you to protect her.