As females in the family have got the disorder it cannot be sex linked. That leaves autosomal recessive (AR) and autosomal dominant(AD). Whether or not an illness is AR or AD depends on what the faulty gene actually does. Every gene in our body (other than those on the sex chromosomes) exists in a pair-one of the pair is from mum and one from dad. What is meant by AR is BOTH the pair have to be faulty to cause the illness. With AD only one of the pair needs the fault for someone to get ill.
A good example of AR is cystic fibrosis. The faulty gene codes for a salt transporting protein which fails to work resulting in very salty sweat and sticky mucus which bungs up the lungs causing breathing problems. In this situation if only one of the pair of genes (alleles) is faulty the person is OK as the other gene makes the salt transporters. It is only when both genes are faulty and none of these salt transporters get made that the illness results.
With AD diseases one copy of the faulty gene normally produces a protein which disrupts cell structure or function. The presence of one healthy copy does not stop the illness occurring.
Obviously all relatives share genes. This goes beyond just the basics such as hair and eye colour. In fact a lot of our behviour and temperament is geneticlally determined (although environment clearly also contributes here). First degree relatives are parents/children and brothers/sisters. We share 50% of genes with our first degree relatives. Second degree relatives are our aunties and uncles. We share 25% genetic material with them. Cousins are third degree relatives. They share 12.5% (or one -eighth) of genes.
When cousins have children this can cause problems. As they share genes they are likely to share autosomal recessive defects. Both the related parents will not be unwell as they only have one copy of the defective gene each. However when they have children there is a 1 in 4 chance that a child will get both bad copies and be unwell. Unfortunately in your husbands family there has been some bad luck as 3 out of 4 children have inherited both copies of the faulty gene when statistically only one out of 4 should have got it. Your husband will have 2 bad copies of the gene.
As you and your husband are not blood related, and as you have no history of blindness in your biological relatives, the chances you happen to have exactly one copy of the same defective gene is very very small. Therefore with your children, 50% will have one copy of the bad gene and 50% will have no bad copies. However with an AR disease one copy of the faulty gene will not cause illness, but will make that child a carrier of the condition. So based on the statistics, half your children are carriers and half are totally unaffected.
The implications of this is that there is always a chance in the future that one of your children could have a child with RP if they have children with another carrier. Chances are this person will have a history of RP in the family. However even if they do have RP in the family this does not automatically mean there is a risk as there are over 100 genes which can cause RP. Everyone affected in the same family will have the same gene, but 2 different affected families usually have a different faulty gene.
So basically if you have more children it is highly unlikely they will have RP. With your children 50% have no increased risk of having children with the condition, but the other 50% being carriers need to be careful if they choose to have children with someone else who has RP in the family.