It sounds like everyone in your household is stressed out...I can understand the strain it must put on everyone.

I know from the experience of my brother being a single parent for a year (he has since gotten back together with his wife) and him living with my mom what a strain it put on my mom, who has a very hard time saying "no," as in "No, I don't want to watch him tonight while you do so and so." Eric and I ended up babysitting quite a bit, not for my brother- but for my mom, who was taking on way too much.

I guess my best advice is, for your own sake, as long as she isn't putting the baby in any kind of danger, don't be afraid to step back and let her handle things...my mom aged a lot last year from feeling like she needed to do too much, when in fact it was my brother's responsibility. I know your daughter is only 18, and you all help out which is great...just make sure you take some time for yourself.