Mere ghar pe my MIL told me to sit with a straight back and use a hot water bag. Honestly, a warm compress and resting as much as possible is the best natural cure. Just avoid lifting heavy things.
My tracking app explained that the pregnancy hormone 'relaxin' loosens your ligaments, causing back pain. It also suggested doing light prenatal stretches which helped me a lot. Do check with your doctor before trying any exercises though.
Your body is going through so many changes to protect your precious little one, so back pain is very natural. Please be very gentle with yourself and don't strain. If the pain gets too sharp, do let your doctor know.
With twins, my back felt like it belonged to a 90-year-old! Your body is carrying so much extra weight, so backache is bound to happen. Try sitting with good back support and make your husband massage it!