This is good advice @Rekha gupta. I was about to suggest getting a bowl of warm water and a flannel/rag as I do this for my dad, but sometimes when the need is more urgent, the same baby wipes is used instead. I'm not sure why you have found them unhelpful.When you clean your mum do it while she is in bed. Use a bowl of water and a sponge, or cloth to clean her and dry her on one side, gently turn her over and clean her on the other side, then finally place her on her back to clean her private area. It is best to keep a separate cloth to clean that area. Watch out for pressure sores which look like small red marks usually on the heels, elbows, spine or bottom. These, if left, become blisters and becomes an ulcer. Once this happens it needs medical attention, but you can help to prevent it by moving her in her bed several times a day (ideally every 2 hours) - turning her over and placing her in different positions - and as soon as you start to see a red mark use barrier cream, the sort used on babies bottoms, 2 or 3 times a day.
It sounds like she would need to sit in a reclining chair - one where the back is tipped back rather than upright. In UK I would say that you need a hoist to move her as you may damage your back, but Im guessing that you cant get hold of one.
If you don't have a reclining chair, perhaps cushions may help instead.