I'd broach the issue as gently as possible, with him definitely and maybe her.
How long have they been together?
I was with my partner for not very long when I fell pregnant, and then went into labour 6 weeks early.
For the next 4 years, I always felt the in laws hated me, couldn't really work out why, maybe because I fell pregnant very quickly..
Any way, after finding out yet again that my in laws had been slagging me off, OH went to confront them. Turns out they didn't think my twins were my partners, thought I'd gotten pg to my ex (impossible by the way, MASSIVE gap between him and my partner, over a year! I've NEVER cheated, nor given any reason to think I have?! I was and still am totally besotted with this guy soppy I know)
Horrible time! I'm still having trouble getting over it now, 5 years later.
We did a DNA test and proved they were partners, but it's now left things awkward, all the time.
Which is why I say talk to her, I'd rather my MIL had spoken to me about it while pg or when twins were born, I would have understood to a degree. We were young, I'd fallen pg very early in the relationship, I'd given birth to twins of healthy weights 6 weeks early, and she had been listening to the bile my ex was giving her.
If she had just spoken to me, and explained her reasons, I would have had the DNA test done, with much less animosity than there is now, I am I think anyway a reasonable woman.
Can you speak to her do you think? Explain why you would like the DNA test done, but if you don't talk to her, then I suggest you don't behave as my MIL did, don't be nasty about her, it WILL get back to her, don't treat the grandchild with any negativity. That's my advice for what it's worth