Adeline a.k.a. Widow Jenkins is a sweet old lady who helps Rachel out when she can. She sees no other option and is super matter-of-fact about pitching in. Check out what she says to Rachel about lending a hand with Jacob:
"I helped raise you so I can help with this one too," Widow Jenkins said, "but only if you wait till this boy's a year old. That way he'll be proper weaned. I won't take no pay for keeping him either." (7.39)
She's kind and no-nonsense, and importantly, one of the only people Rachel has to help her get by. Because of this, her murder is especially heartbreaking. You could argue that Buchanan and Wilkie do their fair share of shady deals—after all, they're in it for the money just as much as the Pembertons—so when they die, it's a bummer but not a tragedy. Widow Jenkins, on the other hand, is sweet and selfless, so when she gets the axe we know Serena is growing more wicked and manipulative than ever.
R.I.P. Widow Jenkins.