In the case of "altruistic" family arrangements I wonder how easy it would be to refuse?
A sister might make the offer to her sibling and she may be 100% certain that is able to do it, having had a child or even completed her family or perhaps having never been pregnant before. She is of sound mind and thinks she can do it. The sibling accepts and all is fine, there are no problems with the pregnancy and the baby is born healthy, the sister recovers well from the birth and they all live happily. Or something somehow goes wrong. The offer itself could cause a problem in the sibling relationship, there could be difficult getting pregnant, there could be a miscarriage/s resulting in deeply emotional issues with pain and physical risks and there could even be a question over a disabled foetus, extreme morning sickness, regret, issues with a husband or partner who has to do more with existing children whilst the woman struggles physically in late pregnancy and maybe even the wish to terminate the pregnancy.
Granted, this was between friends and not between sisters but it's still relevant in terms of altruistic surrogacy. Would it be different - better or worse - if this was between sisters?
https://www.mumsnet.com/Talk/womens_rights/4239144-Surrogacy-ends-in-abortion-and-ends-a-friendship
It could also be that a sibling asks their sister. She will have seen their sibling struggle with infertility for years, perhaps there was a childhood disease or condition that meant their sibling would always struggle to conceive and the sister had been waiting the question for years too. Maybe she has felt the burden of it and had it as an expectation of her and is resigned and feels she can't say no. There are numerous scenarios and each one is individual.
There is an argument to say that as commercial surrogacy is a transaction it is a simpler arrangement. Of course it would be disingenuous to say there were no emotions involved at all but once contracts are signed it could be said that the process is relatively straightforward as both parties agree on diet, having sex, kissing, breastmilk etc. This is harder to navigate in families I think.
https://www.reddit.com/r/relationshipadvice/comments/hr29qg/myysisteranddbilwonttletmeekissmyy_bf/
I've read many a story about surrogacy in families and many of them work well and all the adults are fine, the children were all too young to comment and won't know any different, whether their grandmother has given birth to them and pumped breastmilk for the first 6 months of life, or maybe their aunt is also their mother and the egg that made them came from elsewhere. I do find that, in terms of coercion, it's possible that surrogacy within families is more likely to happen. I have read about the mother of two sisters and how proud she was of the eldest daughter being so kind and I can't imagine it would be easy for that woman to say she has doubts or a change of heart. I've also heard about family surrogacy in families with a strong faith and strict cultures where the women as subservient or serve a purpose. It's very complex and no one family is exactly the same as any other so naturally a surrogacy within a family could only be compared to another on basic principles. They are not the same.
For the record, I do think surrogacy works for many many couples, the surrogate-born children may also be just fine, but as we saw with adopted children and see more with donor conceived child more recently, some are fine and others are not. It isn't about the majority. We don't say, well the majority of adopted children are fine so let's not worry about the few who do suffer genealogical bewilderment and have mental health issues as a result. We try to help them, learn from them, see how potentially problems can be avoided and even build regulatory and legal frameworks to keep them safe.