It's so wearying when the pressure of caring for an aging parent gets tangled up with old family tensions, and those harsh texts cut deep, especially when everyone is already hurting. The stress of dementia caregiving often amplifies whatever cracks already exist in a family, and what you're describing is a heavy load to carry day after day. You're wise to be asking God to send someone who can step into the middle of this as a steady, calming presence.
When things are this raw, sometimes the most immediate step is simply to protect the channels of communication that can stay healthy. If the texts are landing on your husband's phone, it might help to set a temporary boundary, not to cut his sister off permanently, but to give everyone a chance to let the temperature come down before anything else lands. A brief, neutral message like "We both want what's best for Mom, let's talk when things are calmer" can sometimes break the cycle, even if only for a day. If there's a neutral person the family still trusts, a pastor, a longtime family friend, even a geriatric care manager, your husband could quietly ask them to help carry some of the communication, especially around decisions for his mom's care. That's not going behind anyone's back; it's bringing in the peacemaker you're already praying for.
We've seen how easily dementia care becomes a pressure cooker for families, and the strain you're feeling isn't a sign that you're doing anything wrong. It's simply hard. We're praying for you and your husband to have a wisdom that goes beyond this moment's anger, and for the sister's heart to soften enough that the focus can return where it belongs, on his mom and the care she needs.
Lord, in Jesus' name, we ask You to quiet this storm. Shield this couple from the daily stings of conflict and give them Your stubborn peace. Raise up someone who can speak calmly into this family, and help them make decisions for Mom with clarity and compassion rather than resentment. Strengthen this wife with rest that goes deeper than sleep, and let her see even a small glimmer of what You are doing in the middle of this. Amen.