Tissue valves tend to last fewer years in young people like your sister than they do in the usual valve patient, who is older. It's my impression, although I don't know if there has been any research done on this, that the valve lifetime is not as predictable in a young person as in someone older, so that makes the decision even more complicated than it might be. She won't necessarily know how long a tissue valve will last her.
If your sister wants to have children, the tissue valve is a better choice, but she may need to go ahead and have her children quickly, in case the valve life turns out to be only a few years. The only way I am aware that a woman with a mechanical valve might be able to carry a pregnancy to term is by taking daily heparin injections for anticoagulation purposes. Most people with a mechanical heart valve take warfarin tablets, but warfarin is contraindicated in pregnancy. Heparin is supposed to be safe for the baby, but there is no oral heparin; it has to be injected.
If I were in your sister's place, and I could live with the thought of not having children, I believe I would go ahead and get a mechanical valve. If your sister gets a tissue valve at her age, then even if it does last her 10 to 15 years, she will require at least one more open-heart surgery, and OHS is a heavy-duty surgery. There is a small but real percentage of people who do not live through open-heart surgery. FWIW, I've been through two valve replacement surgeries myself, and I never want to go through another one.
I'm female, but I was past child-bearing age when I had the first surgery, so having children was not a concern for me, but I think it all comes down to that: how badly does your sister want to have children? Because if I knew I wanted children, I would get the tissue valve. Then when the tissue valve had to be replaced, that's when I would get the mechanical valve. Best wishes to your sister as she makes this decision and goes through her surgery.
A tissue valve will normally last only for 10-15 years. Mechanical valves are generally appropriate for a majority of patients 65 years old and younger
The choice for a 26 year old person would normally be not so difficult.
There is however a new procedure evolving very quickly with which it is possible to repair valves via a catheter (TAVI procedure). It is to be expected that surgeons will be able to repair a defect prosthetic tissue valve through such a procedure within 15 years.
Mild MR in itself is not a problem, only if it becomes moderate or severe it needs attention. Often the regurgitation does not increase for many years. Why does the doctor expect worsening of the regurgitation?