Resentment while caring for a parent

The suttas rank caring for parents very high and they never pretend it is easy; the same canon that calls parents Brahmā also names resentment as something that arises, is fed, and can be put down. Minding the cost is not a failure of love. The passages here give resentment specific things to do instead of being swallowed. What they do not do is divide the labor among your siblings or tell you when enough is enough; those are conversations for the people involved, and for a doctor when your own health is in it.

What the suttas say

“Mendicants, a mendicant should use these five ways to completely get rid of resentment when it has arisen toward anyone. What five? You should develop love for an individual you resent. That’s how to get rid of resentment for that individual. You should develop compassion for an individual you resent. … You should develop equanimity for an individual you resent. … You should disregard an individual you resent, paying no attention to them. … You should focus on the ownership of deeds regarding that individual: ‘This venerable is the owner of their deeds and heir to their deeds. Deeds are their womb, their relative, and their refuge. They shall be the heir of whatever deeds they do, whether good or bad.’ That’s how to get rid of resentment for that individual. A mendicant should use these five ways to completely get rid of resentment when it has arisen toward anyone.”
A practical list — love, compassion, equanimity, not attending, reflecting on kamma.
Where this stops: Reflecting that people own their deeds is never a way to call someone's misfortune their kamma, and putting down resentment is never a reason to stay near someone who keeps doing harm.
AN 5.161:1.1–1.15 · Getting Rid of Resentment (1st)
Mendicants, a family where the children honor their parents in their home is said to live with divinity. A family where the children honor their parents in their home is said to live with the first tutors. A family where the children honor their parents in their home is said to live with those worthy of offerings dedicated to the gods. ‘Divinity’ is a term for your parents. ‘First tutors’ is a term for your parents.
On caring for parents.
Where this stops: It assumes parents who raised and cared for their children; never use it to press anyone back into contact, or into care they cannot give.
AN 3.31:1.1–1.5 · With Divinity
And how do you look after others by looking after yourself? By development, cultivation, and practice of meditation. And how do you look after yourself by looking after others? By acceptance, harmlessness, love, and sympathy.
The acrobats — the case that solitary practice is not selfish, and that care for others is itself practice.
Where this stops: For the worry that time given to one's own practice is taken from the people one cares for; not for a decision about having a child, and not a ruling on how to care for someone who is ill.
SN 47.19:3.1–3.5 · At Sedaka
“Do you then serve them with fresh and cooked foods and savories?” “Sometimes I do.” “But if they don’t accept it, brahmin, who does it belong to?” “In that case it still belongs to me.” In the same way, brahmin, when you abuse, harass, and attack us who do not abuse, harass, and attack, we don’t accept it. It still belongs to you, brahmin, it still belongs to you! Someone who, when abused, harassed, and attacked, abuses, harasses, and attacks in return is said to eat the food and have a reaction to it. But we neither eat your food nor do we have a reaction to it.
Declining to take in what someone hurls at you in anger, the way a guest can leave a dish untouched.
Where this stops: Concerns how abuse is received inwardly; it is no reason to stay where someone keeps abusing you.
SN 7.2:2.5–3.2 · With Bhāradvāja the Rude
Even as a mother would protect with her life her child, her only child, so too for all creatures unfold a boundless heart. With love for the whole world, unfold a boundless heart: above, below, all round, unconstricted, without enmity or foe. When standing, walking, sitting, or lying down while yet unweary, keep this ever in mind; for this, they say, is a divine meditation in this life.
The Metta Sutta's central image, in the canon's own words.
Where this stops: The verses are an ideal of the practitioner; the opening lines describing a skilled and honest person are a description, not a prerequisite for beginning. Never use it to quiet anger that has something to say, and never present it as a recitation that protects anyone. The mother guarding her only child is not for someone after a pregnancy loss.
Kp 9:7.1–9.4 · The Discourse on Love

Bring your own situation.