So sorry to hear how your ILs are acting, being a new mum is hard enough when everyone is being supportive, and it must be horrible when they are making these cruel, goady comments.
You are doing the best thing for your baby, you are feeding him the healthiest way. He is growing perfectly. On-demand breastfeeding is supported by medical consensus, WHO, UNICEF etc. Don't let your ILs personal opinions influence your thinking! In parenting, there will always be someone who disagrees with you, and it really isn't worth trying to change their opinion, you just need to disengage from discussing with them.
Saying that, the way they are criticising your decisions and SIL making PA comments, "my baby has two chins", is awful. This needs to stop immediately. I think DH should take the lead on this, and tell his family very clearly in no uncertain terms that they must not make any comments of this nature anymore, as they are cruel and upsetting. He doesn't need to convince them of your point of view, just to tell them that these comments are unacceptable, that you have made your decision how to feed your baby, he completely supports you.
If any more comments are made, calmly say "I disagree, lets talk about something else". If they persist, calmly say you are leaving the room/ going home/ the visit is over. Then DH can follow-up by reiterating that they must not make comments like this.
If someone still carries on, then I would have DH say that you won't see that person as they are persistently acting in a cruel way, and when they change their behaviour they can see you both again. There is no reason why you should keep seeing people who consistently say cruel things to you and refuse to change their behaviour at any time, particularly not when you are feeling vulnerable as a new mother.
It's sad that their behaviour is leading to you becoming anxious and weighing your baby every 1-2 weeks. Sometimes babies' growth stops briefly e.g. if they are unwell, and I worry that if your baby isn't gaining weight one week you will doubt yourself.
The advice to meet other breastfeeding mums is great, if you don't know any locally there are breastfeeding facebook groups e.g. breastfeeding yummy mummies, breastfeeding and parenting support, where you can see that other mums are facing similar difficulties with their families!
Also this We saw her refusing the final 50 ml of her bottle but her parents kept pushing it back in her mouth saying she won't sleep through other wise. The more she refused and pushed it out the more they kept putting it back in saying no bottle goes unfinished. is horrible, force feeding someone is awful, and it sounds like their baby is being over-fed and may be overweight. It's normal for babies' appetites to vary day-by-day, e.g. depending on how active they are, growth spurts, developmental leaps. Does your SIL see a HV, doctor, anyone who is monitoring her baby's growth? Given how she is acting with you, I don't think it is your role to change her thinking, but when she starts boasting about her baby's weight gain perhaps you could say once "have you spoken to your doctor about this, as my Dr said it is usually healthiest to stay on the same centile, perhaps you should get their opinion?"