YABU. I understand your anxieties, however, you don't actually need to know, nor do you have a right to know.
It also isn't as cut and dry as "if your niece has a genetic condition there is a significant chance your child will have it".
I have had genetic testing, and the consultant explained that genetic conditions generally fall into three categories. Autosomal Dominant, Autosomal Recessive and De Novo.
Autosomal Dominant: These genetic disorders only require one copy of a mutated gene to cause a child to be affected by a genetic disorder. If a parent has an autosomal Dominant condition or carries the gene, then there is a 50% chance their child will be affected and a 50% chance they will be a carrier. With an autosomal condition you tend to see at least one person per generation be affected by that condition due to the fact anyone born to a carrier will either have the condition or carry it.
Autosomal Recessive: These disorders need two copies of a mutated gene to be passed on, one from each parent. This means both parents must either carry the mutated gene or have the disorder themselves. If both parents are affected by the mutated gene then there is a 25% chance the child will be affected, a 25% chance a child will be unaffected and a 50% chance a child will be a carrier.
- If only one parent has an autosomal recessive condition or carries the faulty gene then there is a 50% chance the child will be a carrier and a 50% chance they will be totally unaffected.
De Novo: The child has a random, spontaneous gene mutation that isn't inherited by either parent. This can lead to them being a carrier or having a genetic condition.
The vast majority of genetic conditions are either Autosomal Recessive or De Novo. And if your niece does have a genetic condition, then it likely falls into one of these categories presuming that none of her relatives have similar difficulties to her.
Hypothetically speaking if your niece's disability is caused by an Autosomal Recessive disorder, then there would be a 50% chance that you carry the same gene mutation as your sister carries. However, the chances of your husband also carrying that gene would likely be incredibly low.
If your niece has a De Novo genetic condition then there's a pretty much zero chance your child would have the same condition because that's just luck of the draw.
If you are actively in the process of IVF you can pay for pre-implantation genetic screening of your embryos if you have genuine concern that they will have a genetic disorder. My husband and I need IVF treatment and that's what we plan to do due to our specific family circumstances, because we want to be prepared.
However it is incredibly important to note NOT ALL DISABILITIES OR BIRTH DEFECTS ARE CAUSED BY GENETIC DISORDERS.
Cerebral Palsy is caused by brain damage before, during or after birth. You can't predict how birth will go, so you can't realistically plan to prevent it happening. You also wouldn't even know until after the baby was born, and you wouldn't necessarily find out the severity until they missed milestones.
Spina Bifida, Heart Defects, Cleft lip and/or Pallet and Talipes (clubfoot) are all relatively common non genetic congenital disabilities.
Things like Autism, ADHD, Learning difficulties, learning disabilities and developmental disorders are not inherently genetic, despite the fact they do often have a genetic link, where there is a higher chance of being affected if a parent or sibling is.
And that's without touching on the subject of illness and acquired disabilities.
If you are absolutely adamant you cannot cope with a disabled child, then you should not take the risk of having a child.
If you feel you could cope with a disabled child, but your husband has expressed he would leave you and have nothing to do with his child, then I would advise reassessing your relationship and deciding if you want to spend the rest of your life with someone with that attitude.