You say you're not allowed to say anything and have just got to suck it up . You do realise relationships are 2 way!? Say the same back if thats an acceptable thing to say in your relationship with each other. If you don't think that's an acceptable thing to say tell her that stating you will not tolerate being spoken to in that way.
BOUNDARIES- I agree with earlier poster.
1) you are a responsible adult- decide what standard of grand parenting and time and financial commitment you are able to offer your daughter and inform her. You are responsible for your own choices.
2) Suggest to your daughter she makes a claim to Child Maintenance Service Gov.uk to claim financial support from the father of her children
3) If the children have health needs ensure they have a health assessment and a social worker assessment to identify support needs, educational needs (SEND?) and support services available and any potential £benefits that may be possible to claim
4) If your daughter does not work she is available full time to care for her own children. If she is unable to cope she needs to inform her GP and SW accordingly. It is not your responsibility to become a second parent or carer to your daughter. If you feel you have become a carer ensure you have your own carer assessment completed and whether entitlement to Carers Allowance. You would be doing your daughter a favour to acknowledge her lack of ability to cope with her children and seek all the assistance and support that may be available as a result.
5) The grandmother in the Tik Tok video has pumped up filled lips, appears to have had facial botox, has hair colour, full make up and maybe even a face lift as her face looks somewhat plastic. Not a look I aspire to but the point being she takes time and spends money to look after her own needs and appearance as well as being a grandmother. Tell your daughter you are taking her advice to "look after your appearance". Book yourself a massage, relaxing spa da, manicure, new haircut, and few new clothes and take some time out to relax and care for yourself. This will demonstrate to your daughter that you value yourself!. If you don't value and respect yourself your daughter won't either!
The fact the Tik Tok video has upset you shows how defensive and vulnerable you have become. Seek your own support through your GP if the situation is damaging your health and well being. If you have the resilience and feel able- take the tough love steps outlined above. Do you have a friend or relative who can help and support you during your tough love steps towards your own recovery?
There are many Tik Tok videos out there also of Trad Wives, looking glamorous, baking cakes and making jam, looking after multiple children seemingly unaided. Send one to your daughter with a smile saying how you can't believe everything you see online these days and not everyone lives a charmed life in reality. Wonder allowed with how much money does she think the posters make out of their tin Tok videos/ what's true and make believe etc. I.e talk about them together rather than react emotionally to them.
You and your daughter live in this world, not their world. It's possible to change the current situation with some adjustments on both sides. Your life- your choice.
Take care and good luck.