Horrific, and I don't say that lightly.
'Murder In-Law' is truly awful. It's interesting to note that this was released almost two months prior to 'Killer Grandma', also known as 'Killer In-Law'. That latter title is not the only similarity, as the majority of these films feels incredibly similar - perhaps an 'Antz'/'A Bug's Life' type of situation?
I thoroughly enjoyed that other production, which tackles a similar premise but executes the plot way better; mainly thanks to Nana Visitor's performance. This misses a big performer, all of the acting is pretty poor and adds nothing. The writing et al. is just as bad.
The characters act so stupidly, they all lack any awareness entirely. The whole viewing experience is painful, I was begging for the end credits from the very early stages, despite the short ~90 minute run time. Despite the dark nature of the story, the film shies away from most of the would-be entertaining/eye-catching moments.
Like the concept? Check out 'Killer Grandma'. Otherwise, I'd suggest avoiding... unless you want to witness this in all its 'glory', of course.